qqplot

David Winsemius <dwinsemius at comcast.net> wrote

I always assumed that the intercept was zero and the slope = unity.

 y <- rt(200, df = 5)
 qqnorm(y); qqline(y, col = 2)
 qqplot(y, rt(300, df = 5))
 abline(0, 1, col="red")

Suppose you have the following

x <- rnorm(500)
y <- 500*(x + runif(500, 0,1))
qqplot(x,y)


Then an abline(0,1) will not be useful; it will be an almost horizontal line. But

m1 <- lm(y~x)
abline(coef(m1))


does the trick.

Peter



Peter L. Flom, PhD
Statistical Consultant
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