Minimal example of collider bias, using R

mwe-collider.R

set.seed(42)

dat <- data.frame(
  A = rbinom(500, 1, 0.5),
  B = rbinom(500, 1, 0.5)
)
dat$C = rbinom(500, 1, 0.1 + 0.3*dat$A + 0.4*dat$B)

tb <- with(dat, table(A,B))
tb0 <- with(subset(dat, C==0), table(A,B))
tb1 <- with(subset(dat, C==1), table(A,B))

print(tb)
print(chisq.test(tb))
print(tb0)
print(chisq.test(tb0))
print(tb1)
print(chisq.test(tb1))