[R] nested ANCOVA: still confused

Doran, Harold HDoran at air.org
Tue Jan 24 21:04:21 CET 2006

Dear Jeff:

I see the issues in your code and have provided what I think will solve
your problem. It is often much easier to get help on this list when you
provide a small bit of data that can be replicated and you state what
the error messages are that you are receiving. OK, with that said, here
is what I see. First, you do not need to use the syntax bb$sex in your
model, this can be significantly simplified. Second, you do not have a
random statement in your model.

Here is your original model:
lme(bb$rtot~bb$sex, bb$purban|bb$chick/bb$box, na.action=na.omit)

Here is what it should be:

lme(fixed = rtot~sex, random=~purban|chick/box, na.action=na.omit,

Notice there is a fixed and random call. You can simplify this as

lme(rtot~sex, random=~purban|chick/box, na.action=na.omit, bb)

Note, you can eliminate the "fixed=" portion but not the random

Last, if you want to do this in lmer, the newer function for mixed
models in the Matrix package, you would do

lmer(rtot~sex + (purban|box:chick) + (purban|box), na.action=na.omit,

Hope this helps.

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Jeffrey Stratford
Sent: Tuesday, January 24, 2006 11:34 AM
To: r-help at stat.math.ethz.ch
Subject: [R] nested ANCOVA: still confused

Dear R-users,

I did some more research and I'm still not sure how to set up an ANCOVA
with nestedness.  Specifically I'm not sure how to express chicks nested
within boxes.  I will be getting Pinheiro & Bates (Mixed Effects Models
in S and S-Plus) but it will not arrive for another two weeks from our
interlibrary loan.

The goal is to determine if there are urbanization (purban) effects on
chick health (rtot) and if there are differences between sexes (sex) and
the effect of being in the same clutch (box).

The model is rtot = sex + purban + (chick)box.

I've loaded the package lme4.  And the code I have so far is

bb <- read.csv("C:\\eabl\\eabl_feather04.csv", header=TRUE) bb$sex <-
factor(bb$sex) rtot.lme <- lme(bb$rtot~bb$sex,

but this is not working.

Any suggestions would be greatly appreciated.



Jeffrey A. Stratford, Ph.D.
Postdoctoral Associate
331 Funchess Hall
Department of Biological Sciences
Auburn University
Auburn, AL 36849
FAX 334-844-9234

R-help at stat.math.ethz.ch mailing list
PLEASE do read the posting guide!

More information about the R-help mailing list