inst/snippet/fit-bugs-pois01.R

In fastR2: Foundations and Applications of Statistics Using R (2nd Edition)

o <- c(2, 10, 16, 11, 5, 3, 3)
o.collapsed <- c(2 + 10, 16, 11, 5, 3 + 3)
n <- sum(o)
m <- sum(o * 0:6) / n     # mean count = MLE for lambda (full data)
p <- dpois(0:6, m)  
p.collapsed <- c(p[1] + p[2], p[3:5], 1 - sum(p[1:5]))   # collapsed probs
e.collapsed <- p.collapsed * n
cbind(o.collapsed, p.collapsed, e.collapsed)
lrt  <- 2 * sum(o.collapsed * log(o.collapsed / e.collapsed)); lrt
pearson <- sum((o.collapsed - e.collapsed)^2 / e.collapsed); pearson
1-pchisq(lrt, df = 3)
1-pchisq(pearson, df = 3)