pp.n <- 20
pp <- data.frame(p=exp(-5*runif(pp.n)))
pp$x <- log(pp$p/(1-pp$p))
limits <- c(5,25,75,95,100) / 100
pp$score <- rep(0,pp.n)
pp$level <- factor(pp$score)
for (i in 1:length(limits))
	pp$score[pp$p>limits[i]] <- i + 1
pp$y <- 100+pp$x + 4*rnorm(pp.n)
pp.mean <- tapply(pp$y,pp$score,mean)

pp.x <- aov(y~x,pp)
pp.xs <- aov(y~score+x,pp)

pp.s <- aov(y~score,pp)
pp.score <- aov(y~score+level,pp)

pp.p <- aov(y~p,pp)