glm coefficients
In your example there is not categorical variables and you need at least one
for ANCOVA.
Try the following, is the same dataset using factor() to create a
categorical variable (named CatVar):
dat <- data.frame(response = rnorm(9), size = rnorm(9), CatVar =
factor(c(1,1,1,1,0,0,0,0,0)))
model <- glm(response ~ size * CatVar, data = dat)
coef(model)
You will get:
(Intercept) size CatVar1 size:CatVar1
0.2141371 -0.7847063 -1.8409264 3.3637699
The first coefficient (labeled "Intercept") is the intercept for CatVar = 0.
The second coefficient (labeled "size") is the slope for CatVar=0
The third coefficient (labeled "CatVar1") is the difference between the
intercept for CatVar = 0 and the intercept for CatVar = 1
The fourth coefficient ("labeled size:CatVar") is the difference between the
slope for CatVar = 0 and the slope for CatVar = 1
You can check it plotting this:
plot(dat$size,dat$response,col=dat$CatVar)
abline(a=coef(model)[1],b=coef(model)[2],col=1)
abline(a=sum(coef(model)[c(1,3)]),b=sum(coef(model)[c(2,4)]),col=2)
This way of presenting coefficients is the default in R (treatment contrast)
but there are other alternatives, see ?contr.treatment.
Hope it helps,
Aitor
--------------------------------------------------
From: "Scott Chamberlain" <scttchamberlain4 at gmail.com>
Sent: Friday, September 23, 2011 5:59 PM
To: <R-sig-ecology at r-project.org>
Subject: [R-sig-eco] glm coefficients
Hello, Is there a way to add up coefficients from a glm model for an ANCOVA to get the coefficients for each term? For example, in the following: dat <- data.frame(response = rnorm(9), size = rnorm(9), covariate = c(1,1,1,1,0,0,0,0,0)) model <- glm(response ~ size * covariate, data = dat)
coef(model)
(Intercept) size covariate size:covariate -0.2995964 -0.1969266 0.7158756 1.2829886 we are interested in the coefficients for the intercepts for size at both levels of the covariate (0 and 1), and the slopes for each line. This requires adding up coefficients from the above output. Are there built in base functions or in other packages that do these additions? Thanks! __ Scott Chamberlain [[alternative HTML version deleted]]
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