I have just uploaded a new version of the R package agrmt to R-Forge. The package implements various measures to enumerate the degree of agreement, consensus, or polarization among respondents. Apart from van der Eijk’s Agreement “A”, there are a range of other measures proposed in the literature.
Same Explanatory Variables, Multiple Dependent Variables in R
I needed to run variations of the same regression model: the same explanatory variables with multiple dependent variables. In R, we can do this with a simple for() loop and assign().
First I specify the dependent variables:
dv <- c("dv1", "dv2", "dv3")
Then I create a for() loop to cycle through the different dependent variables:
for(i in 1:length(dv)){
Within this loop, I need to create an object to hold the models. I need a separate object for each model, so I create one with paste(). For the first dependent variable, this will be model1; for the second dependent variable model2, and so on.
model <- paste("model",i, sep="")
With this object to hold the model in place, I can run the model: the ith dependent variable is used. It is stored in an object called m.
m <- lm(as.formula(paste(dv[i],"~ ev1 + ev2")), data=mydata)
Now, I assign the model m to the model object created above: model1 for the first dependent variable, etc. That’s also the end of the for() loop.
assign(model,m)}
We can now look at the results:
summary(model1); summary(model2); summary(model3)
or, more practical to compare models:
library(memisc)
mtable(model1, model2, model3)
Barplots across variables in R
Here’s a good example of how useful sapply can be. I have some data from Qualtrics, and each response is coded in its own variable. Let’s say there is a question on what kind of organization respondents work in, with 10 response categories. Qualtrics produces 10 variables, each with 1 if the box was ticked, and empty otherwise (structure shown just below). With the default CSV import, these blank cells are turned into NA. Here’s a simple way to produce a barplot in this case (in R, of course).
types = sapply(1:10, function(i) sum(get(paste("Q1_",i,sep="")), na.rm=TRUE))
barplot(types)
Let’s take this step by step. To count frequencies, we simply use sum(), with the argument na.rm=TRUE because the variables only contain 1 and NA. get() is used to find the variable specified by a string; the string is created with paste(). In this case, the variable names are Q1_1, Q1_2, Q1_3, … Q1_9, Q1_10. By using paste(), we combine the “Q1_” part with the counter variable i, with no separation (sep="").
The whole thing is then wrapped up in sapply(), with the counter variable i defined to take values from 1 to 10; the function(i) part is there so that the counter variable is applied to the sum. So sapply() takes each value of the counter variable, and applies it to the function we specified, which calculates the sum for one variable Q1_i at a time.
Now I can simply do a boxplot, and add the names.arg argument to specify the labels.
(Here I specified the colours: barplot(types, col=rainbow(10)) to have a catchy image at the top of this post, albeit one where colours have no meaning: so-called chart-junk).
Didier Ruedin 