Is it possible to store the summary of an eRm model? - r

Question

I am using the eRm package to estimate a Rasch model. The RM() function returns a Rasch model that I can summarize using the summary() function. However, when I try to store the results, R creates an empty object.
library(eRm)
my_data <- matrix(sample(0:1, 100, replace = TRUE), nrow = 10)
my_model <- RM(X = my_data)
summary(my_model)
my_summary <- summary(my_model)
Why does this operation not work in this case but does work when storing the summary of a linear model? Is there another way to store the summary of the eRm model?

Answer 1

As #Imo surmised, it looks like summary.eRm just prints to the console, rather than returning an object. You can inspect the code for summary.eRm by running getAnywhere(summary.eRm). summary is a "generic" function, meaning that what it does depends on what "method" is called when the function is invoked.
For an lm model object, when you type summary(my_model), the summary.lm function is dispatched. But when you type summary(my_model) and my_model is an eRm object, the summary.eRm method is dispatched. summary.lm returns an object, but summary.eRm just prints to the console. Run methods(summary) to see the various summary functions that get dispatched for different types of objects.
A workaround would be to create your own summary object (or a function to create such an object), using the model object itself. You can inspect the components of the model object with str(my_model). You can also look at the code for summary.eRm to see where it is getting each of the components that it prints to the console.
Here's a simple example, lifting code from summary.eRm to create a summary function:
RMsmry = function(obj) {
cols = c("Estimate", "Std. Error", "lower CI", "upper CI")
# Create difficulty summary
ci = confint(obj, "eta")
tbl1 = as.data.frame(cbind(round(obj$etapar, 3),
round(obj$se.eta, 3), round(ci, 3)))
names(tbl1) = cols
# Create easiness summary
ci <- confint(obj, "beta")
tbl2 = as.data.frame(cbind(round(obj$betapar, 3),
round(obj$se.beta, 3), round(ci, 3)))
names(tbl2) = cols
return(list(Difficulty=tbl1, Easiness=tbl2))
}
my_summary = RMsmry(my_model)
my_summary
$Difficulty
Estimate Std. Error lower CI upper CI
I2 -1.191 0.658 -2.480 0.098
I3 -1.191 0.658 -2.480 0.098
I4 0.078 0.627 -1.150 1.306
I5 -0.750 0.623 -1.971 0.471
I6 0.078 0.627 -1.150 1.306
I7 1.079 0.748 -0.386 2.544
I8 -0.339 0.614 -1.543 0.865
I9 0.078 0.627 -1.150 1.306
I10 1.079 0.748 -0.386 2.544
$Easiness
Estimate Std. Error lower CI upper CI
beta I1 -1.079 0.748 -2.544 0.386
beta I2 1.191 0.658 -0.098 2.480
beta I3 1.191 0.658 -0.098 2.480
beta I4 -0.078 0.627 -1.306 1.150
beta I5 0.750 0.623 -0.471 1.971
beta I6 -0.078 0.627 -1.306 1.150
beta I7 -1.079 0.748 -2.544 0.386
beta I8 0.339 0.614 -0.865 1.543
beta I9 -0.078 0.627 -1.306 1.150
beta I10 -1.079 0.748 -2.544 0.386