Cook's distance

y <- (1:100)^2
x <- 1:100
lm(y~x) -> linmod # just for the sake of a simple example

cooks.distance(linmod)[1]

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of kolos.agoston at gmx.net
Sent: Tuesday, March 12, 2013 3:55 PM
To: r-help at r-project.org
Subject: [R] Cook's distance

Dear useRs,

I have some trouble with the calculation of Cook's distance in R.
The formula for Cook's distance can be found for example here:
http://en.wikipedia.org/wiki/Cook%27s_distance

I tried to apply it in R:

y <- (1:400)^2
x <- 1:100
lm(y~x) -> linmod # just for the sake of a simple example

linmod$residuals[1]^2/(2*mean(linmod$residuals^2))*(hatvalues(linmod)[1
]/(1-hatvalues(linmod)[1])^2)
         1
0.02503195

cooks.distance(linmod)[1]

         1
0.02490679

Why differ the two results?

Thanks a lot if somebody have some instructions for me.

Best wishes:

Kolos