Different output from lm() and lmPerm lmp() if categorical variables are included in the analysis
RTFM!!! :-)
The help explicitly says "The default contrasts are set internally to
(contr.sum, contr.poly) ....".
Set
options(contrasts=c("contr.sum","contr.poly"))
before your call to lm() and "atest" will agree with "aptest" all down
the line.
cheers,
Rolf Turner
On 11/08/13 21:35, Agustin Lobo wrote:
I've found a problem when using
categorical variables in lmp() from package lmPerm
According to help(lmp): "This function will behave identically to lm()
if the following parameters are set: perm="", seq=TRUE,
center=FALSE.")
But not in the case of including categorical variables:
require(lmPerm)
set.seed(42)
testx1 <- rnorm(100,10,5)
testx2 <- c(rep("a",50),rep("b",50))
testy <- 5*testx1 + 3 + runif(100,-20,20)
test <- data.frame(x1=testx1,x2=
testx2,y=testy)
atest <- lm(y ~ x1*x2,data=test)
aptest <- lmp(y ~ x1*x2,data=test,perm = "", seqs = TRUE, center = FALSE)
summary(atest)
Call:
lm(formula = y ~ x1 * x2, data = test)
Residuals:
Min 1Q Median 3Q Max
-17.1777 -9.5306 -0.9733 7.6840 22.2728
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -2.0036 3.2488 -0.617 0.539
x1 5.3346 0.2861 18.646 <2e-16 ***
x2b 2.4952 5.2160 0.478 0.633
x1:x2b -0.3833 0.4568 -0.839 0.404
summary(aptest)
Call:
lmp(formula = y ~ x1 * x2, data = test, perm = "", seqs = TRUE,
center = FALSE)
Residuals:
Min 1Q Median 3Q Max
-17.1777 -9.5306 -0.9733 7.6840 22.2728
Coefficients:
Estimate Std. Error t value Pr(>|t|)
x1 5.1429 0.2284 22.516 <2e-16 ***
x21 -1.2476 2.6080 -0.478 0.633
x1:x21 0.1917 0.2284 0.839 0.404
It looks like lmp() is internally coding dummy variables in a different way, so
lmp results are for "a" (named "1" by lmp) while lm results are for
"b" ?