Categories

## Less wordy R

The Swarm Lab presents a nice comparison of R and Python code for a simple (read ‘one could do it in Excel’) problem. The example works, but I was surprised by how wordy the R code was and decided to check if one could easily produce a shorter version.

The beginning is pretty much the same, although I’ll use ggplot2 rather than lattice, because it will be a lot easier (and shorter) to get the desired appearance for the plots:

require(Quandl)
require(ggplot2)

# Load data from Quandl
my.data = Quandl("TPC/HIST_RECEIPT",
start_date = "1945-12-31",
end_date = "2013-12-31")



The whole example relies on only three variables and—as I am not great at typing—I tend to work with shorter variable names. I directly changed the names for variables 1 to 3:

# Display first lines of the data frame
# and set short names for first three columns
names(my.data)[1:3] = c('year', 'indtax', 'corptax')


It is a lot easier to compare the regression lines if we change the shape of the data set from wide to long, where there is one variable for year, one for tax type, and one for the actual tax rate. It would be possible to use one of Hadley’s packages to get a simpler syntax for this, but I decided to stick to the minimum set of requirements:

# Change shape to fit both regressions simultaneously
mdlong = reshape(my.data[, 1:3],
idvar = 'year', times = c('Individual', 'Corporate'),
varying = list(2:3), direction = 'long')

mdlong$taxtype = factor(mdlong$time)


And now we are ready to produce the plots. The first one can be a rough cut to see if we get the right elements:

ggplot(mdlong, aes(x = year, y = indtax, color = taxtype)) +
geom_point() + geom_line() + geom_smooth(method = 'lm')


Yes, this one has the points, lines, linear regression and 95% confidence intervals for the mean predicted responses, but we still need to get rid of the grey background and get black labels (theme_bw()), set the right axis labels and ticks (scale_x... scale_y...) and set the right color palette for points and lines (scale_colour_manual) and filling the confidence intervals (scale_colour_fill) like so:

# Plotting the graph (first match color palette) and put the regression
# lines as well
serious.palette = c('#AD3333', '#00526D')
ggplot(mdlong, aes(x = year, y = indtax, color = taxtype)) +
geom_point() + geom_line() + geom_smooth(method = 'lm', aes(fill = taxtype)) +
theme_bw() +
scale_y_continuous('Income taxes (% of GDP)', breaks = seq(0, 12, 2), minor_breaks = NULL) +
scale_x_date('Fiscal year', minor_breaks = NULL) +
scale_colour_manual(values=serious.palette) + scale_fill_manual(values=serious.palette)


One can still change font sizes to match the original plots, reposition the legend, change the aspect ratio while saving the png graphs (all simple statements) but you get the idea. If now we move to fitting the regression lines:

# Fitting a regression with dummy variables
m1 = lm(indtax ~ year*taxtype, data = mdlong)
summary(m1)

# The regressions have different intercepts and slopes
# Call:
#   lm(formula = indtax ~ year * taxtype, data = mdlong)
#
# Residuals:
#   Min       1Q   Median       3Q      Max
# -1.95221 -0.44303 -0.05731  0.35749  2.39415
#
# Coefficients:
#                            Estimate Std. Error t value Pr(>|t|)
#   (Intercept)             3.435e+00  1.040e-01   33.01   <2e-16 ***
#   year                   -1.564e-04  1.278e-05  -12.23   <2e-16 ***
#   taxtypeIndividual       4.406e+00  1.471e-01   29.94   <2e-16 ***
#   year:taxtypeIndividual  1.822e-04  1.808e-05   10.08   <2e-16 ***
#   ---
#   Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
#
# Residual standard error: 0.7724 on 134 degrees of freedom
# Multiple R-squared:  0.9245,  Adjusted R-squared:  0.9228
# F-statistic: 546.9 on 3 and 134 DF,  p-value: < 2.2e-16


This gives the regression coefficients for Corporate (3.45 - 1.564e-04 year) and Individual ([3.45 + 4.41] + [-1.564e-04 + 1.822e-04] year or 7.84 + 2.58e-05 year). As a bonus you get the comparison between regression lines.

In R as a second language I pointed out that 'brevity reduces the time between thinking and implementation, so we can move on and keep on trying new ideas'. Some times it seriously does.

Categories

## R as a second language

Imagine that you are studying English as a second language; you learn the basic rules, some vocabulary and start writing sentences. After a little while, it is very likely that you’ll write grammatically correct sentences that no native speaker would use. You’d be following the formalisms but ignoring culture, idioms, slang and patterns of effective use.

R is a language and any newcomers, particularly if they already know another programming language, will struggle at the beginning to get what is beyond the formal grammar and vocabulary. I use R for inquisition: testing ideas, data exploration, visualization; under this setting, the easiest is to perform a task the more likely is one going to do it. It is possible to use several other languages for this but—and I think this is an important but—R’s brevity reduces the time between thinking and implementation, so we can move on and keep on trying new ideas.

A typical example is when we want to repeat something or iterate over a collection of elements. In most languages if one wants to do something many times the obvious way is using a loop (coded like, for() or while()). It is possible to use a for() loop in R but many times is the wrong tool for the job, as it increases the lag between thought and code, moving us away from ‘the flow’.

# Generate some random data with 10 rows and 5 columns
M = matrix(round(runif(50, 1, 5), 0), nrow = 10, ncol = 5)
M

#      [,1] [,2] [,3] [,4] [,5]
# [1,]    2    3    4    2    1
# [2,]    3    1    3    3    4
# [3,]    4    2    5    1    3
# [4,]    2    4    4    5    3
# [5,]    2    3    1    4    4
# [6,]    3    2    2    5    1
# [7,]    1    3    5    5    2
# [8,]    5    4    2    5    4
# [9,]    3    2    3    4    3
#[10,]    4    4    1    2    3

# Create dumb function that returns mean and median
# for data
sillyFunction = function(aRow) {
c(mean(aRow), median(aRow))
}

# On-liner to apply our function to each row
apply(M, 1, sillyFunction)

#     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
#[1,]  2.4  2.8    3  3.6  2.8  2.6  3.2    4    3   2.8
#[2,]  2.0  3.0    3  4.0  3.0  2.0  3.0    4    3   3.0

# or one could do it for each column
apply(M, 2, sillyFunction)

# Of course one could use a loop. Pre-allocating
# the result matrix would have a loop with little
# time penalty (versus growing the matrix)
nCases = dim(M)[1]
resMatrix = matrix(0, nrow = nCases, ncol = 2)
# and here is the loop
for(i in 1:nCases){
resMatrix[i, 1:2] = sillyFunction(M[i,])
}

resMatrix
# Same results as before
#      [,1] [,2]
# [1,]  2.4    2
# [2,]  2.8    3
# [3,]  3.0    3
# [4,]  3.6    4
# [5,]  2.8    3
# [6,]  2.6    2
# [7,]  3.2    3
# [8,]  4.0    4
# [9,]  3.0    3
#[10,]  2.8    3


One of the distinctive features of R is that there is already a lot of functionality available for jobs that occur frequently in data analysis. The easiest is to perform a task the more likely is one going to do it, which is perfect if one is exploring/thinking about data.

Thomas Lumley reminded me of the ACM citation for John Chambers—father of S of which R is an implementation—which stated that Chambers’s work:

…will forever alter the way people analyze, visualize, and manipulate data . . . S is an elegant, widely accepted, and enduring software system, with conceptual integrity, thanks to the insight, taste, and effort of John Chambers.

If I could summarize the relevance of R in a Tweetable phrase (with hash tags and everything) it would be:

Most data analysis languages underestimate the importance of interactivity/low barrier to exploration. That’s where #Rstats shines.

One could run statistical analyses with many languages (including generic ones), but to provide the right level of interactivity for analysis, visualization and data manipulation one ends up creating functions that, almost invariably, look a bit like R; pandas in Python, for example.

There are some complications with some of the design decisions in R, especially when we get down to consistency which begets memorability. A glaring example is the apply family of functions and here is where master opportunist (in the positive sense of expert at finding good opportunities) Hadley Wickham made sense out of confusion in his package plyr.

There is also a tension in languages under considerable use because speakers/writers/analysts/coders start adapting them to new situations, adding words and turns of phrase. Look at English for an example! This is also happening to R and some people wish the language looked different in some non-trivial ways. A couple of examples: Coffeescript for R and Rasmus Bååth’s suggestions. Not all of them can be implemented, but suggestions like this speak of the success of R.

If you are struggling to start working with R, as with other languages, first let go. The key to learning and working with a new language is immersing yourself in it; even better if you do it with people who already speak it.

Just to be clear, there are several good statistical languages. However, none is as supportive of rapid inquisition as R (IMO). It is not unusual to develop models in one language (e.g. R) and implement it in another for operational purposes (e.g. SAS, Python, whatever).

The first thing I admire about Hadley is his ‘good eye’ for finding points of friction. The second one is doing something about the frictions, often with very good taste.

P.S. It should come clear from this post that English is indeed my second language.

Categories

## Teaching linear models

I teach several courses every year and the most difficult to pull off is FORE224/STAT202: regression modeling.

The academic promotion application form in my university includes a section on one’s ‘teaching philosophy’. I struggle with that part because I suspect I lack anything as grandiose as a philosophy when teaching: as most university lecturers I never studied teaching, although I try to do my best. If anything, I can say that I enjoy teaching and helping students to ‘get it’ and that I want to instill a sense of ‘statistics is fun’ in them. I spend quite a bit of time looking for memorable examples, linking to stats in the news (statschat and listening the news while walking my dog are very helpful here) and collecting data. But a philosophy? Don’t think so.

One of the hardest parts of the course is the diversity of student backgrounds. Hitting the right level, the right tone is very hard. Make it too easy and the 1/5 to 1/4 of students with a good mathematical background will hate it; they may even decide to abandon any intention of continuing doing stats if ‘that’s all there is about the topic’. Make it too complicated and half the class will fail and/or hate the content.

Part of the problem is based around what we mean by teaching ‘statistics’. In some cases it seems limited to what specific software does; for example, teaching with Excel means restriction to whatever models are covered in Excel’s Data Analysis Toolpak (DAT). The next choice when teaching is using menu-driven software (e.g. SPSS), which provides much more statistical functionality than Excel + DAT, at the expense of being further removed from common usability conventions. At the other extreme of simplicity is software that requires coding to control the analyses (e.g. R or SAS). In general, the more control we want, the more we have to learn to achieve it.

A while ago I made a distinction between the different levels of learning (user cases) when teaching statistics. In summary, we had i- very few students getting in to statistics and heavy duty coding, ii- a slightly larger group that will use stats while in a while and iii- the majority that will mostly consume statistics. I feel a duty towards the three groups, while admitting that I have predilection for the first one. Nevertheless, the third group provides most of the challenges and need for thinking about how to teach the subject.

When teaching linear models (general form $$y = X eta + epsilon$$) we tend to compartmentalize content: we have an ANOVA course if the design matrix $$X$$ represents categorical predictors (contains only 1s and 0s), a regression course if $$X$$ is full of continuous predictors and we talk about ANCOVA or regression on dummy variables if $$X$$ is a combination of both. The use of different functions for different contents of $$X$$ (for example aov() versus lm() in R or proc reg versus proc glm in SAS) further consolidates the distinction. Even when using menus, software tends to guide students through different submenus depending on the type of $$X$$.

At the beginning of the course we restrict ourselves to $$X$$ full of continuous predictors, but we introduce the notion of matrices with small examples. This permits showing the connection between all the linear model courses (because a rose by any other name…) and it also allows deriving a general expression of the formulas for the regression coefficients (essential for the most advanced students). Slower students may struggle with some of this material; however, working with small examples they can replicate the results from R (or Excel or SAS or whatever one uses to teach). Some times they even think it is cool.

Here is where the model.matrix() R function becomes handy; rather than building incidence matrices by hand—which is easy for tiny examples—we can get the matrices used by the lm() function to then calculate regression parameters (and any other output) for more complex models.

Once students get the idea that on matrix terms our teaching compartments are pretty much the same, we can reinforce the idea by using a single function (or proc) to show that we can obtain all the bits and pieces that make up what we call ‘fitting the model’. This highlights the idea that ANOVA, ANCOVA & regression are subsets of linear models, which are subsets of linear mixed models, which are subsets of generalized linear mixed models. A statistical Russian doll.

We want students to understand, some times so badly that we lower the bar to a point where there is no much to understand. Here is the tricky part, finding the right level of detail so all types of students learn to enjoy the topic, although at different levels of understanding.

There is software that generates code from menus too, like Stata or Genstat.

P.S. This is part of my thinking aloud with hesitation about teaching, as in Statistics unplugged, Excel, fanaticism and R, Split-plot 1: How does a linear mixed model look like?, R, academia and the democratization of statistics, Mid-January flotsam: teaching edition & Teaching with R: the switch. I am always looking for better ways of transferring knowledge.

Categories

## Statistics unplugged

How much does statistical software help and how much it interferes when teaching statistical concepts? Software used in the practice of statistics (say R, SAS, Stata, etc) brings to the party a mental model that it’s often alien to students, while being highly optimized for practitioners. It is possible to introduce a minimum of distraction while focusing on teaching concepts, although it requires careful choice of a subset of functionality. Almost invariably some students get stuck with the software and everything goes downhill from there; the student moved from struggling with a concept to struggling with syntax (Do I use a parenthesis here?).

I am a big fan of Tim Bell’s Computer Science Unplugged, a program for teaching Computer Science’s ideas at primary and secondary school without using computers (see example videos).

Here is an example video for public key encryption:

This type of instruction makes me question both how we teach statistics and at what level we can start teaching statistics. The good news is that the New Zealand school curriculum includes statistics in secondary school, for which there is increasing number of resources. However, I think we could be targeting students even earlier.

This year my wife was helping primary school students participating in a science fair and I ended up volunteering to introduce them to some basic concepts so they could design their own experiments. Students got the idea of the need for replication, randomization, etc based on a simple question: Did one of them have special powers to guess the result of flipping a coin? (Of course this is Fisher’s tea-drinking-lady-experiment, but no 10 year old cares about tea, while at least some of them care about super powers). After the discussion one of them ran a very cool experiment on the effect of liquefaction on the growth of native grasses (very pertinent in post-earthquake Christchurch), with 20 replicates (pots) for each treatment. He got the concepts behind the experiment; software just entered the scene when we needed to confirm our understanding of the results in a visual way:

People tell me that teaching stats without a computer is like teaching chemistry without a lab or doing astronomy without a telescope, or… you get the idea. At the same time, there are some books that describe some class activities that do not need a computer; e.g. Gelman’s Teaching Statistics: A Bag of Tricks. (Incidentally, why is that book so friggin’ expensive?)

## Back to uni

Back from primary school kiddies to a regression course at university. Let’s say that we have two variables, x & y, and that we want to regress y (response) on x (predictor) and get diagnostic plots. In R we could simulate some data and plot the relationship using something like this:

# Basic regression data
n = 100
x = 1:n
y = 70 + x*5 + rnorm(n, 0, 40)

# Changing couple of points and plotting
y[50] = 550
y[100] = 350
plot(y ~ x)


We can the fit the linear regression and get some diagnostic plots using:

# Regression and diagnostics
m1 = lm(y ~ x)

par(mfrow = c(2,2))
plot(m1)
par(mfrow = c(1,1))


If we ask students to explain the 4th plot—which displays discrepancy (how far a point is from the general trend) on leverage (how far is a point from the center of mass, pivot of the regression)—many of them will struggle to say what is going on in that plot. At that moment one could go and calculate the Hat matrix of the regression ($$X (X’X)^{-1} X’$$) and get leverage from the diagonal, etc and students will get a foggy idea. Another, probably better, option is to present the issue as a physical system on which students already have experience. A good candidate for physical system is using a seesaw, because many (perhaps most) students experienced playing in one as children.

Take your students to a playground (luckily there is one next to uni), get them playing with a seesaw. The influence of a point is related to the product of leverage (how far from the pivot we are applying force) and discrepancy (how big is the force applied). The influence of a point on the estimated regression coefficients will be very large when we apply a strong force far from the pivot (as in our point y[100]), just as it happens in a seesaw. We can apply lots of force (discrepancy) near the pivot (as in our point [y[50]) and little will happen. Students like mucking around with the seesaw and, more importantly, they remember.

Analogy can go only so far. Some times a physical analogy like a quincunx (to demonstrate the central limit theorem) ends up being more confusing than using an example with variables that are more meaningful for students.

I don’t know what is the maximum proportion of course content that could be replaced by using props, experiments, animations, software specifically designed to make a point (rather than to run analysis), etc. I do know that we still need to introduce ‘proper’ statistical software—at some point students have to face praxis. Nevertheless, developing an intuitive understanding is vital to move from performing monkeys; that is, people clicking on menus or going over the motion of copy/pasting code without understanding what’s going on in the analyses.

I’d like to hear if you have any favorite demos/props/etc when explaining statistical concepts.

P.S. In this post I don’t care if you love stats software, but I specifically care about helping learners who struggle understanding concepts.

Categories

## Using Processing and R together (in OS X)

I wanted to develop a small experiment with a front end using the Processing language and the backend calculations in R; the reason why will be another post. This post explained the steps assuming that one already has R and Processing installed:

1. Install the Rserve package. This has to be done from source (e.g. using R CMD INSTALL packagename).
2. Download Rserve jar files and include them in the Processing sketch.
3. Run your code

For example, this generates 100 normal distributed random numbers in R and then sorts them (code copy and pasted from second link):

import org.rosuda.REngine.Rserve.*;
import org.rosuda.REngine.*;

double[] data;

void setup() {
size(300, 300);

try {
RConnection c = new RConnection();
// generate 100 normal distributed random numbers and then sort them
data= c.eval("sort(rnorm(100))").asDoubles();

} catch ( REXPMismatchException rme ) {
rme.printStackTrace();

} catch ( REngineException ree ) {
ree.printStackTrace();
}
}

void draw() {
background(255);
for( int i = 0; i < data.length; i++) {
line( i * 3.0, height/2, i* 3.0, height/2 - (float)data[i] * 50 );
}
}


The problem is that this didn't work, because my OS X (I use macs) R installation didn't have shared libraries. My not-so-quick solution was to compile R from source, which involved:

1. Downloading R source. I went for the latest stable version, but I could have gone for the development one.
2. Setting up the latest version of C and Fortran compilers. I did have an outdated version of Xcode in my macbook air, but decided to delete it because i- uses many GB of room in a small drive and ii- it's a monster download. Instead I went for Apple's Command Line Tools, which is a small fraction of size and do the job.
3. In the case of gfortran, there are many sites pointing to this page that hosts a fairly outdated version, which was giving me all sorts of problems (e.g. "checking for Fortran 77 name-mangling scheme") because the versions between the C and Fortran compilers were out of whack. Instead, I downloaded the latest version from the GNU site.
4. Changing the config.site file in a few places, ensuring that I had:
5. CC="gcc -arch x86_64 -std=gnu99"
CXX="g++ -arch x86_64"
F77="gfortran -arch x86_64"
FC="gfortran -arch x86_64"


Then compiled using (didn't want X11 and enabling shared library):

./configure --without-x --enable-R-shlib
make
make check
make pdf # This produces a lot of rubbish on screen and it isn't really needed
make info


And finally installed using:

sudo make prefix=/luis/compiled install

This used a prefix because I didn't want to replace my fully functioning R installation, but just having another one with shared libraries. If one types R in terminal then it is still calling the old version; the new one is called via /luis/compiled/R.framework/Versions/Current/Resources/bin/R. I then installed Rserve in the new version and was able to call R from processing so I could obtain.

Now I can move to what I really wanted to do. File under stuff-that-I-may-need-to-remember-one-day.

Categories

## Excel, fanaticism and R

This week I’ve been feeling tired of excessive fanaticism (or zealotry) of open source software (OSS) and R in general. I do use a fair amount of OSS and pushed for the adoption of R in our courses; in fact, I do think OSS is a Good ThingTM. I do not like, however, constant yabbering on why using exclusively OSS in science is a good idea and the reduction of science to repeatability and computability (both of which I covered in my previous post). I also dislike the snobbery of ‘you shall use R and not Excel at all, because the latter is evil’ (going back ages).

We often have several experiments running during the year and most of the time we do not bother setting up a data base to keep data. Doing that would essentially mean that I would have to do it, and I have a few things more important to do. Therefore, many data sets end up in… (drum roll here) Microsoft Excel.

How should a researcher setup data in Excel? Rather than reinventing the wheel, I’ll use a(n) (im)perfect diagram that I found years ago in a Genstat manual.

I like it because:

• It makes clear how to setup the experimental and/or sampling structure; one can handle any design with enough columns.
• It also manages any number of traits assessed in the experimental units.
• It contains metadata in the first few rows, which can be easily skipped when reading the file. I normally convert Excel files to text and then I skip the first few lines (using skip in R or firstobs in SAS).

People doing data analysis often start convulsing at the mention of Excel; personally, I deeply dislike it for analyses but it makes data entry very easy, and even a monkey can understand how to use it (I’ve seen them typing, I swear). The secret for sane use is to use Excel only for data entry; any data manipulation (subsetting, merging, derived variables, etc.) or analysis is done in statistical software (I use either R or SAS for general statistics, ASReml for quantitative genetics).

It is far from a perfect solution but it fits in the realm of the possible and, considering all my work responsibilities, it’s a reasonable use of my time. Would it be possible that someone makes a weird change in the spreadsheet? Yes. Could you fart while moving the mouse and create a non-obvious side effect? Yes, I guess so. Will it make your life easier, and make possible to complete your research projects? Yes sir!

P.S. One could even save data using a text-based format (e.g. csv, tab-delimited) and use Excel only as a front-end for data entry. Other spreadsheets are of course equally useful.

P.S.2. Some of my data are machine-generated (e.g. by acoustic scanners and NIR spectroscopy) and get dumped by the machine in a separate—usually very wide; for example 2000 columns—text file for each sample. I never put them in Excel, but read them directly (a directory-full of them) in to R for manipulation and analysis.

As an interesting aside, the post A summary of the evidence that most published research is false provides a good summary for the need to freak out about repeatability.

Categories

## Flotsam 13: early July links

Man flu kept me at home today, so I decided to do something ‘useful’ and go for a linkathon:

Sometimes people are truthful and cruel. Here Gappy on a mission goes for the jugular:

Over and out.

Categories

## My take on the USA versus Western Europe comparison of GM corn

A few days ago I came across Jack Heinemann and collaborators’ article (Sustainability and innovation in staple crop production in the US Midwest, Open Access) comparing the agricultural sectors of USA and Western Europe. While the article is titled around the word sustainability, the main comparison stems from the use of Genetically Modified crops in USA versus the absence of them in Western Europe.

I was curious about part of the results and discussion which, in a nutshell, suggest that “GM cropping systems have not contributed to yield gains, are not necessary for yield gains, and appear to be eroding yields compared to the equally modern agroecosystem of Western Europe”. The authors relied on several crops for the comparison (Maize/corn, rapeseed/canolasee P.S.6, soybean and cotton); however, I am going to focus on a single one (corn) for two reasons: 1. I can’t afford a lot of time for blog posts when I should be preparing lectures and 2. I like eating corn.

When the authors of the paper tackled corn the comparison was between the USA and Western Europe, using the United Nations definition of Western Europe (i.e. Austria, Belgium, France, Germany, Liechtenstein, Luxembourg, Monaco, Netherlands, Switzerland). Some large European corn producers like Italy are not there because of the narrow definition of Western.

I struggled with the comparison used by the authors because, in my opinion, there are potentially so many confounded effects (different industry structures, weather, varieties, etc.) that it can’t provide the proper counterfactual for GM versus non-GM crops. Anyway, I decided to have a look at the same data to see if I would reach the same conclusions. The article provides a good description of where the data came from, as well as how the analyses were performed. Small details to match exactly the results were fairly easy to figure out. I downloaded the FAO corn data (3.7 MB csv file) for all countries (so I can reuse the code and data later for lectures and assignments). I then repeated the plots using the following code:

# Default directory
setwd('~/Dropbox/quantumforest')

# Required packages
require(ggplot2)
require(labels)

# Reading FAO corn data

# Extracting Area
FAOarea = subset(FAOcorn, Element == 'Area Harvested',
select = c('Country', 'Year', 'Value'))

names(FAOarea)[3] = 'Area'

# and production
FAOprod = subset(FAOcorn, Element == 'Production',
select = c('Country', 'Year', 'Value'))

names(FAOprod)[3] = 'Production'

# to calculate yield in hectograms
FAOarea = merge(FAOarea, FAOprod, by = c('Country', 'Year'))
FAOarea$Yield = with(FAOarea, Production/Area*10000) # Subsetting only the countries of interest (and years to match paper) FAOarticle = subset(FAOarea, Country == 'United States of America' | Country == 'Western Europe') # Plot with regression lines ggplot(FAOarticle, aes(x = Year, y = Yield, color = Country)) + geom_point() + stat_smooth(method = lm, fullrange = TRUE, alpha = 0.1) + scale_y_continuous('Yield [hectograms/ha]', limits = c(0, 100000), labels = comma) + theme(legend.position="top")  I could obtain pretty much the same regression model equations as in the article by expressing the years as deviation from 1960 as in: # Expressing year as a deviation from 1960, so results # match paper FAOarticle$NewYear = with(FAOarticle, Year - 1960)

usa.lm = lm(Yield ~ NewYear, data = FAOarticle,
subset = Country == 'United States of America')
summary(usa.lm)

#Call:
#lm(formula = Yield ~ NewYear, data = FAOarticle, subset = Country ==
#    "United States of America")
#
#Residuals:
#     Min       1Q   Median       3Q      Max
#-18435.4  -1958.3    338.3   3663.8  10311.0
#
#Coefficients:
#            Estimate Std. Error t value Pr(>|t|)
#(Intercept) 38677.34    1736.92   22.27|t|)
#(Intercept) 31510.14    1665.90   18.91

Heinemann and collaborators then point out the following:

…the slope in yield increase by year is steeper in W. Europe (y?=?1344.2x?+?31512, R²?=?0.92084) than the United States (y?=?1173.8x?+?38677, R²?=?0.89093) from 1961 to 2010 (Figure 1). This shows that in recent years W. Europe has had similar and even slightly higher yields than the United States despite the latter’s use of GM varieties.

However, that interpretation using all data assumes that both ‘countries’ are using GMO all the time. An interesting thing is that USA and Western Europe were in different trends already before the introduction of GM corn. We can state that because we have some idea of when GM crops were introduced in the USA. This information is collected by the US Department of Agriculture in their June survey to growers and made publicly available at the State level (GMcornPenetration.csv):

cornPenetration = read.csv('GMcornPenetration.csv')

ggplot(cornPenetration, aes(x = Year, y = PerAllGM)) + geom_line() + facet_wrap(~ State) +
scale_y_continuous('Percentage of GM corn') +
theme(axis.text.x  = theme_text(angle=90))


This graph tells us that by the year 2000 the percentage of planted corn was way below 50% in most corn producing states (in fact, it was 25% at the country level). From that time on we have a steady increase reaching over 80% for most states by 2008. Given this, it probably makes sense to assume that, at the USA level, yield reflects non-GM corn until 1999 and progressively reflects the effect of GM genotypes from 2000 onwards. This division is somewhat arbitrary, but easy to implement.

We can repeat the previous analyzes limiting the data from 1961 until, say, 1999:

usa.lm2 = lm(Yield ~ NewYear, data = FAOarticle,
subset = Country == 'United States of America' & Year < 2000)
summary(usa.lm2)

#Call:
#lm(formula = Yield ~ NewYear, data = FAOarticle, subset = Country ==
#    "United States of America" & Year < 2000)
#
#Residuals:
#   Min     1Q Median     3Q    Max
#-17441  -2156   1123   3989   9878
#
#Coefficients:
#            Estimate Std. Error t value Pr(>|t|)
#(Intercept) 39895.57    2084.81   19.14  < 2e-16 ***
#NewYear      1094.82      90.84   12.05 2.25e-14 ***
#---
#Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
#
#Residual standard error: 6385 on 37 degrees of freedom
#Multiple R-squared:  0.797,	Adjusted R-squared:  0.7915
#F-statistic: 145.2 on 1 and 37 DF,  p-value: 2.245e-14

weu.lm2 = lm(Yield ~ NewYear, data = FAOarticle,
subset = Country == 'Western Europe' & Year < 2000)
summary(weu.lm2)

#Call:
#lm(formula = Yield ~ NewYear, data = FAOarticle, subset = Country ==
#    "Western Europe" & Year < 2000)
#
#Residuals:
#   Min     1Q Median     3Q    Max
#-10785  -3348    -34   3504  11117
#
#Coefficients:
#            Estimate Std. Error t value Pr(>|t|)
#(Intercept) 29802.17    1813.79   16.43

These analyses indicate that Western Europe started with a lower yield than the USA (29,802.17 vs 39,895.57 hectograms/ha) and managed to increase yield much more quickly (1,454.48 vs 1,094.82 hectograms/ha per year) before any use of GM corn by the USA. Figure 1 shows a messy picture because there are numerous factors affecting yield each year (e.g. weather has a large influence). We can take averages for each decade and see how the two ‘countries’ are performing:

# Aggregating every decade.
# 2013-07-05 20:10 NZST I fixed the aggregation because it was averaging yields rather
# calculating total production and area for the decade and then calculating average yield
# Discussion points are totally valid
FAOarticle$Decade = cut(FAOarticle$Year,
breaks = seq(1959, 2019, 10),
labels = paste(seq(1960, 2010, 10), 's', sep = ''))

decadeProd = aggregate(Production ~ Country + Decade,
data = FAOarticle,
FUN = sum)

decadeArea = aggregate(Area ~ Country + Decade,
data = FAOarticle,
FUN = sum)

decadeYield$Yield = with(decadeYield, Production/Area*10000) ggplot(decadeYield, aes(x = Decade, y = Yield, fill = Country)) + geom_bar(stat = 'identity', position = 'dodge') + scale_y_continuous('Yield [hectograms/ha]', expand = c(0, 0)) + theme(legend.position="top")  This last figure requires more attention. We can again see that Western Europe starts with lower yields than the USA; however, it keeps on increasing those yields faster than USA, overtaking it during the 1990s. Again, all this change happened while both USA and Western Europe were not using GM corn. The situation reverses in the 2000s, when the USA overtakes Western Europe, while the USA continuously increased the percentage of GM corn. The last bar in Figure 3 is misleading because it includes a single year (2010) and we know that yields in USA went down in 2011 and 2012, affected by a very large drought (see Figure 4). At least when looking at corn, I can’t say (with the same data available to Heinemann) that there is no place or need for GM genotypes. I do share some of his concerns with respect to the low level of diversity present in staple crops but, in contrast to his opinion, I envision a future for agriculture that includes large-scale operations (either GM or no-GM), as well as smaller operations (including organic ones). I’d like to finish with some optimism looking further back to yield, because the USDA National Agricultural Statistics Service keeps yield statistics for corn since 1866(!) (csv file), although it uses bizarre non-metric units (bushels/acre). As a metric boy, I converted to kilograms per hectare (multiplying by 62.77 from this page) and then to hectograms (100 g) multiplying by 10. # Reading NASS corn data NASS = read.csv('NASScorn.csv') # Conversion to sensical units (see Iowa State Extension article) # http://www.extension.iastate.edu/agdm/wholefarm/html/c6-80.html NASS$Yield = with(NASS, Value*62.77*10)

# Average by decade
NASS$Decade = cut(NASS$Year,
breaks = seq(1859, 2019, 10),
labels = paste(seq(1860, 2010, 10), 's', sep = ''))

oldYield = aggregate(Yield ~ Decade, data = NASS, FUN = mean)

# Plotting
ggplot(oldYield, aes(x = Decade, y = Yield)) +
geom_bar(stat = 'identity') +
scale_y_continuous('Yield [hectograms]', expand = c(0, 0))


It is interesting to see that there was little change until the 1940s, with the advent of the Green Revolution (modern breeding techniques, fertilization, pesticides, etc.). The 2010s decade in Figure 4 includes 2010, 2011 and 2012, with the last two years reflecting extensive droughts. Drought tolerance is one of the most important traits in modern breeding programs.

While Prof. Heinemann and myself work for the same university I don’t know him in person.

P.S. Did you know that Norman Borlaug (hero of mine) studied forestry?
P.S.2 Time permitting I’ll have a look at other crops later. I would have liked to test a regression with dummy variables for corn to account for pre-2000 and post-1999, but there are not yet many years to fit a decent model (considering natural variability between years). We’ll have to wait for that one.
P.S.3 I share some of Heinemann’s concerns relating to subsidies and other agricultural practices.
P.S.4 In case anyone is interested, I did write about a GM-fed pigs study not long ago.
P.S.5 2013-07-05 20:10 NZST. I updated Figures 1 and 3 to clearly express that yield was in hectograms/ha and recalculated average decade yield because it was originally averaging yields rather calculating total production and area for the decade and then calculating average yield. The discussion points I raised are still completely valid.
P.S.6 2013-07-07 11:30 NZST. The inclusion of Canadian canola does not make any sense because, as far as I know, Canada is not part of Western Europe or the US Midwest. This opens the inclusion of crops from any origin as far as the results are convenient for one’s argument.
P.S.7 2014-08-06 13:00 NZST My comment was expanded and published in the journal (or here for HTML version with comments on the authors’ reply to my comment.

Categories

## Flotsam 12: early June linkathon

A list of interesting R/Stats quickies to keep the mind distracted:

• A long draft Advanced Data Analysis from an Elementary Point of View by Cosma Shalizi, in which he uses R to drive home the message. Not your average elementary point of view.
• Good notes by Frank Davenport on starting using R with data from a Geographic Information System (GIS). Read this so you get a general idea of how things fit together.
• If you are in to maps, Omnia sunt Communia! provides many good tips on producing them using R.
• Mark James Adams reminded us that Prediction ? Understanding, probably inspired by Dan Gianola‘s course on Whole Genome Prediction. He is a monster of Bayesian applications to genetic evaluation.
• If you are in to data/learning visualization you have to watch Bret Victor’s presentation on Media for thinking the unthinkable. He is so far ahead what we normally do that it is embarrassing.
• I follow mathematician Atabey Kaygun in twitter and since yesterday I’ve been avidly reading his coverage of the protests in Turkey. Surely there are more important things going on in the world than the latest R gossip.

I’m marking too many assignments right now to have enough time to write something more substantial. I can see the light at the end of the tunnel though.

Categories

## Analyzing a simple experiment with heterogeneous variances using asreml, MCMCglmm and SAS

I was working with a small experiment which includes families from two Eucalyptus species and thought it would be nice to code a first analysis using alternative approaches. The experiment is a randomized complete block design, with species as fixed effect and family and block as a random effects, while the response variable is growth strain (in $$mu epsilon$$).

When looking at the trees one can see that the residual variances will be very different. In addition, the trees were growing in plastic bags laid out in rows (the blocks) and columns. Given that trees were growing in bags siting on flat terrain, most likely the row effects are zero.

Below is the code for a first go in R (using both MCMCglmm and ASReml-R) and SAS. I had stopped using SAS for several years, mostly because I was running a mac for which there is no version. However, a few weeks ago I started accessing it via their OnDemand for Academics program via a web browser.

The R code using REML looks like:

# Options
options(stringsAsFactors = FALSE)
setwd('~/Dropbox/research/2013/growth-stress')

# Packages
require(ggplot2)
require(asreml)
require(MCMCglmm)

# Reading data, renaming, factors, etc
summary(gs)

# Both argophloia and bosistoana
gsboth = subset(gs, !is.na(strain))
gsboth = within(gsboth, {
species = factor(species)
row = factor(row)
column = factor(column)
fam = factor(fam)
})

ma = asreml(strain ~ species, random = ~ fam + row,
rcov = ~ at(species):units,
data = gsboth)

summary(ma)$varcomp # gamma component std.error z.ratio constraint #fam!fam.var 27809.414 27809.414 10502.036 2.6480022 Positive #row!row.var 2337.164 2337.164 3116.357 0.7499666 Positive #species_E.argopholia!variance 111940.458 111940.458 26609.673 4.2067580 Positive #species_E.bosistoana!variance 63035.256 63035.256 7226.768 8.7224681 Positive  While using MCMC we get estimates in the ballpark by using: # Priors bothpr = list(R = list(V = diag(c(50000, 50000)), nu = 3), G = list(G1 = list(V = 20000, nu = 0.002), G2 = list(V = 20000, nu = 0.002), G3 = list(V = 20000, nu = 0.002))) # MCMC m2 = MCMCglmm(strain ~ species, random = ~ fam + row + column, rcov = ~ idh(species):units, data = gsboth, prior = bothpr, pr = TRUE, family = 'gaussian', burnin = 10000, nitt = 40000, thin = 20, saveX = TRUE, saveZ = TRUE, verbose = FALSE) summary(m2) # Iterations = 10001:39981 # Thinning interval = 20 # Sample size = 1500 # # DIC: 3332.578 # # G-structure: ~fam # # post.mean l-95% CI u-95% CI eff.samp # fam 30315 12211 55136 1500 # # ~row # # post.mean l-95% CI u-95% CI eff.samp # row 1449 5.928 6274 589.5 # # R-structure: ~idh(species):units # # post.mean l-95% CI u-95% CI eff.samp # E.argopholia.units 112017 71152 168080 1500 # E.bosistoana.units 65006 52676 80049 1500 # # Location effects: strain ~ species # # post.mean l-95% CI u-95% CI eff.samp pMCMC # (Intercept) 502.21 319.45 690.68 1500 <7e-04 *** # speciesE.bosistoana -235.95 -449.07 -37.19 1361 0.036 *  The SAS code is not that disimilar, except for the clear demarcation between data processing (data step, for reading files, data transformations, etc) and specific procs (procedures), in this case to summarize data, produce a boxplot and fit a mixed model. * termstr=CRLF accounts for the windows-like line endings of the data set; data gs; infile "/home/luis/Projects/euc-strain/growthstresses.csv" dsd termstr=CRLF firstobs=2; input row column species$ family $strain; if strain ^= .; run; proc summary data = gs print; class species; var strain; run; proc boxplot data = gs; plot strain*species; run; proc mixed data = gs; class row species family; model strain = species; random row family; repeated species / group=species; run; /* Covariance Parameter Estimates Cov Parm Group Estimate row 2336.80 family 27808 species species E.argoph 111844 species species E.bosist 63036 */  I like working with multiple languages and I realized that, in fact, I missed SAS a bit. It was like meeting an old friend; at the beginning felt strange but we were quickly chatting away after a few minutes. Categories ## Subsetting data At School we use R across many courses, because students are supposed to use statistics under a variety of contexts. Imagine their disappointment when they pass stats and discovered that R and statistics haven’t gone away! When students start working with real data sets one of their first stumbling blocks is subsetting data. We have data sets and either they are required to deal with different subsets or there is data cleaning to do. For some reason, many students struggle with what should be a simple task. If one thinks of data as as a matrix/2-dimensional array, subsetting boils down to extracting the needed rows (cases) and columns (variables). In the R world one can do this in a variety of ways, ranging from the cryptic to the explicit and clear. For example, let’s assume that we have a dataset called alltrials with heights and stem diameters for a number of trees in different trials (We may have a bunch of additional covariates and experimental design features that we’ll ignore for the moment). How do we extract all trees located in Christchurch? Two common approaches are: mytrial = alltrials[alltrials$location == 'Christchurch', ]

mytrial = subset(alltrials, location == 'Christchurch')


While both expressions are equivalent, the former reads like Klingon to students, while the latter makes explicit that we are obtaining a subset of the original data set. This can easily be expanded to more complex conditions; for example to include all trees from Christchurch that are taller than 10 m:

mytrial = alltrials[alltrials$location == 'Christchurch' & alltrials$height > 10, ]

mytrial = subset(alltrials, location == 'Christchurch' & height > 10)


I think the complication with the Klingonian notation comes mostly from two sources:

• Variable names for subsetting the data set are not directly accessible, so we have to prefix them with the NameOfDataset$, making the code more difficult to read, particularly if we join several conditions with & and |. • Hanging commas: if we are only working with rows or columns we have to acknowledge it by suffixing or prefixing with a comma, which are often forgotten. Both points result on frustrating error messages like • Error in [.data.frame(alltrials, location == 'Christchurch', ) : object 'location' not found for the first point or • Error in [.data.frame(alltrials, alltrials$location == 'Christchurch') undefined columns selected for the second point.

The generic forms of these two notations are:

dataset[what to do with rows, what to do with columns]

subset(dataset, what to do with rows, what to do with columns)


We often want to keep a subset of the observed cases and keep (or drop) specific variables. For example, we want to keep trees in ‘Christchurch’ and we want to ignore diameter, because the assessor was ‘high’ that day:

# With this notation things get a bit trickier
# The easiest way is to provide the number of the variable
# Here diameter is the 5th variable in the dataset
mytrial = alltrials[all.trials$location == 'Christchurch' & all.trials$height > 10, -5]

# This notation is still straightforward
mytrial = subset(alltrials, location == 'Christchurch' & height > 10, select = -diameter)


There are, however, situations where Klingon is easier or more efficient. For example, to take a random sample of 100 trees from the full dataset:


mytrial = alltrials[sample(1:nrow(alltrials), 100, replace = FALSE),]


If you are interested in this issue Quick-R has a good description of subsetting. I’m sure this basic topic must has been covered many times, although I doubt anyone used Klingon in the process.

Categories

## Learning to code in R

It used to be that the one of the first decisions to make when learning to program was between compiled (e.g. C or FORTRAN) and interpreted (e.g. Python) languages. In my opinion these days one would have to be a masochist to learn with a compiled language: the extra compilation time and obscure errors are a killer when learning.

Today the decision would be between using a generic interpreted language (e.g. Python) and an interpreted domain specific language (DSL) like R, MATLAB, etc. While some people prefer generic languages, I’d argue that immediate feedback and easy accomplishment of useful tasks are a great thing when one is learning something for the first time.

As an example, a while ago my son asked me what I was doing in the computer and I told him that I was programming some analyses in R. I showed that the program was spitting back some numbers and plots, a fact that he found totally unremarkable and uninteresting. I searched in internet and I found Scratch, a visual programming language, that let’s the user moves blocks representing code around and build interactive games: now my son was sold. Together we are learning about loops, control structures and variables, drawing characters, etc. We are programming because the problems are i- much more interesting for him and ii- achievable in a short time frame.

Learning to program for statistics, or other scientific domains for that matter, is not that different from being a kid and learning programming. Having to do too much to get even a mildly interesting result is frustrating and discouraging; it is not that the learner is dumb, but that he has to build too many functions to get a meager reward. This is why I’d say that you should use whatever language already has a large amount of functionality (‘batteries included’ in Python parlance) for your discipline. Choose rightly and you are half-way there.

‘But’ someone will say, R is not a real language. Sorry, but it is a real language (Turing complete and the whole shebang) with oddities, as any other language, granted. As with human languages, the more you study the easier it gets to learn a new language. In fact, the syntax for many basic constructs in R is highly similar to alternatives:

# This is R code
# Loop
for(i in 1:10){
print(i)
}
#[1] 1
#[1] 2
#[1] 3
#[1] 4
#[1] 5
#[1] 6
#[1] 7
#[1] 8
#[1] 9
#[1] 10

# Control
if(i == 10) {
print('It is ten!')
}

#[1] "It is ten!"

# This is Python code
# Loop
for i in range(1,11): # Python starts indexing from zero
print(i)

#1
#2
#3
#4
#5
#6
#7
#8
#9
#10

# Control
if i == 10:
print("It is ten!")

#It is ten!


By the way, I formatted the code to highlight similarities. Of course there are plenty of differences between the languages, but many of the building blocks (the core ideas if you will) are shared. You learn one and the next one gets easier.

How does one start learning to program? If I look back at 1985 (yes, last millennium) I was struggling to learn how to program in BASIC when, suddenly, I had an epiphany. The sky opened and I heard a choir singing György Ligeti’s Atmosphères (from 2001: a Space Odyssey, you know) and then I realized: we are only breaking problems in little pieces and dealing with them one at the time. I’ve been breaking problems into little pieces since then. What else did you expect? Anyhow, if you feel that you want to learn how to code in R, or whatever language is the best option for your problems, start small. Just a few lines will do. Read other people’s code but, again, only small pieces that are supposed to do something small. At this stage is easy to get discouraged because everything is new and takes a lot of time. Don’t worry: everyone has to go through this process.

Many people struggle vectorizing the code so it runs faster. Again, don’t worry at the beginning if your code is slow. Keep on writing and learning. Read Norman Noam Ross’s FasteR! HigheR! StongeR! — A Guide to Speeding Up R Code for Busy People. The guide is very neat and useful, although you don’t need super fast code, not yet at least. You need working, readable and reusable code. You may not even need code: actually, try to understand the problem, the statistical theory, before you get coding like crazy. Programming will help you understand your problem a lot better, but you need a helpful starting point.

Don’t get distracted with the politics of research and repeatability and trendy things like git (noticed that I didn’t even link to them?). You’ll learn them in time, once you got a clue about how to program.

P.S. The code used in the examples could be shortened and sped up dramatically (e.g. 1:10 or range(1, 11)) but it is not the point of this post.
P.S.2. A while ago I wrote R is a language, which could be useful connected to this post.