# Statistics Lab / ferdo — synthetic data (no real student records)
# Save stats_synthetic.csv in your working directory.
d <- read.csv("stats_synthetic.csv", stringsAsFactors = TRUE)
stopifnot(nrow(d) == 36, all(c("score_pre","score_post","group") %in% names(d)))
# Groups are observational labels: differences cannot establish a treatment effect.

tab <- table(d$group, d$preference)
tab
prop.table(tab, margin=1)
test <- chisq.test(tab)
test$expected
test
# Chi-square p-values can be unreliable with small expected counts.