On Aug 23, 2010, at 3:01 PM, Johan Steen wrote:
Dear all,
is there anyone who can help me extracting p-values from an Anova
object from the car library? I can't seem to locate the p-values
using str(result) or str(summary(result)) in the example below
> A <- factor( rep(1:2,each=3) )
> B <- factor( rep(1:3,times=2) )
> idata <- data.frame(A,B)
> fit <- lm( cbind(a1_b1,a1_b2,a1_b3,a2_b1,a2_b2,a2_b3) ˜ sex,
data=Data.wide)
> result <- Anova(fit, type="III", test="Wilks", idata=idata,
idesign=˜A*B)
# you forgot require(car)
> A <- factor( rep(1:2,each=3) )
> B <- factor( rep(1:3,times=2) )
> idata <- data.frame(A,B)
> fit <- lm( cbind(a1_b1,a1_b2,a1_b3,a2_b1,a2_b2,a2_b3)~sex,
data=Data.wide)
Error in inherits(x, "data.frame") : object 'Data.wide' not found
I am guessing that you have an object Data.wide and you are not giving
us any look at it.
Using the lm help page example:
It appears that the print method for Anova is what would return the p-
values:
prtAnova <- Anova(lm.D9 <- lm(weight ~ group), type="III")
> str(prtAnova )
Classes ‘anova’ and 'data.frame': 3 obs. of 4 variables:
$ Sum Sq : num 253.21 0.688 8.729
$ Df : num 1 1 18
$ F value: num 522.13 1.42 NA
$ Pr(>F) : num 9.55e-15 2.49e-01 NA
- attr(*, "heading")= chr "Anova Table (Type III tests)\n"
"Response: weight"
So htis is one way:
> prtAnova$'Pr(>F)'
[1] 9.547128e-15 2.490232e-01 NA
Further examination makes me wonder why you decided that the summary
method did not produce a p-value?
> sumryAnova <- summary(Anova(lm.D9 <- lm(weight ~ group), type="III"))
> str(sumryAnova)
'table' chr [1:7, 1:4] "Min. : 0.6882 " "1st Qu.: 4.7087 "
"Median : 8.7293 " ...
- attr(*, "dimnames")=List of 2
..$ : chr [1:7] "" "" "" "" ...
..$ : chr [1:4] " Sum Sq" " Df" " F value" " Pr(>F)"
Perhaps perhaps you didn't realize that Pr(>F) was the p-value? (It
would be a bit more difficult to get the p-value from the summary
object since it needs to be extracted with attribute functions.
--
David Winsemius, MD
West Hartford, CT
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