On Jan 29, 2011, at 2:36 PM, Joshua Wiley wrote:
On Sat, Jan 29, 2011 at 11:10 AM, David Winsemius
<dwinsem...@comcast.net> wrote:
On Jan 29, 2011, at 12:12 PM, Russell Pierce wrote:
Just in case someone else stumbles onto this thread and is facing a
similar issue: The quick solution for me turned out to be using Glm
and Predict in the rms package. Thanks go to Joshua and Ista for
helping me out with this issue. Double thanks go to Joshua for
suggesting I take a closer look at the rms package.
library(rms)
dat <- data.frame(xxA = rnorm(20,10), xxB = rnorm(20,20))
dat$out <- with(dat,xxA+xxB+xxA*xxB+rnorm(20,20))
rms.res <- Glm(out ~ scale(xxA)*scale(xxB),data=dat)
newdata <-
as.data.frame(Predict(rms.res,xxA=c(-1,0,1),xxB=c(-1,0,1))[,1:3])
Puzzled that you would see a particular need for the rms functions
(not that
I have anything against the rmes package, but it doesn't seem to be
needed
here.)
This seems to work without error:
rms.res <- glm(out ~ scale(xxA)*scale(xxB), data=dat)
newdata <- predict(rms.res,
newdata=data.frame(xxA=c(-1,0,1),xxB=c(-1,0,1)))
The issue is the predicted values are different.
Huh?. With the same model and data, they should be the same:
> lm.mod2 <- lm(out ~ scale(xxA)*scale(xxB), data=dat)
> newdata2 <- expand.grid(xxA=c(-1,0,1),xxB=c(-1,0,1))
> newdata2$preds <- predict(lm.mod, newdata2)
> newdata2
xxA xxB preds
1 -1 -1 218.6366
2 0 -1 232.4330
3 1 -1 246.2295
4 -1 0 230.9129
5 0 0 244.8696
6 1 0 258.8263
7 -1 1 243.1892
8 0 1 257.3062
9 1 1 271.4232
> Predict(rms.res,xxA=c(-1,0,1),xxB=c(-1,0,1), conf.int=FALSE)
xxA xxB yhat
1 -1 -1 218.6366
2 0 -1 232.4330
3 1 -1 246.2295
4 -1 0 230.9129
5 0 0 244.8696
6 1 0 258.8263
7 -1 1 243.1892
8 0 1 257.3062
9 1 1 271.4232
#################################
require(rms)
set.seed(10)
dat <- data.frame(xxA = rnorm(20, 10), xxB = rnorm(20, 20))
dat$out <- with(dat, xxA+xxB+xxA*xxB+rnorm(20,20))
rms.res1 <- Glm(out ~ scale(xxA) * scale(xxB), data=dat)
rms.res2 <- glm(out ~ scale(xxA) * scale(xxB), data=dat)
as.data.frame(Predict(rms.res1, xxA = -1:1, xxB = -1:1))[,1:3]
predict(rms.res2, expand.grid(xxA= -1:1, xxB= -1:1))
##################################
--
David Winsemius, MD
West Hartford, CT
--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/
David Winsemius, MD
West Hartford, CT
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