How to estimate a BTL model in R.

btl_example.R

df <- data.frame(higher = c('US', 'CA', 'MX'),
                 lower = c('CA', 'MX', 'MX'))

levels <- c('US', 'CA', 'MX')
X.l <- model.matrix(~ 0 + factor(higher, levels = levels), data = df)
X.r <- model.matrix(~ 0 + factor(lower, levels = levels), data = df)

X <- X.l - X.r
colnames(X) <- levels # makes it easier to interpret regression output

# The higher one is always better than the lower one.
y <- rep(1, nrow(X))

library(arm)
## Use a slight prior to stabilize estimates.
m <- bayesglm(y ~ 0 + X, family = binomial, prior.scale = 1)
summary(m)