set.seed(1988)
x<-rnorm(30)
y<-rnorm(30)

factor<-cut(x, quantile(x, seq(0,1, 0.25)), right=F, include.lowest=T)
data<-data.frame(x,y,factor)

by(data, data$factor, function(x) cor.test(data$x, data$y))