[R-sig-eco] glm coefficients

[Chris Swan]

I am following this and have two other questions.

1) The first is how to get the correct standard errors for each of the coefficients?  In the example offered, asking for summary(model) will yield, in part, the following (please understand my output will differ given the data generated is random):

Coefficients:
             Estimate Std. Error t value Pr(>|t|)
(Intercept)    0.7988     0.5801   1.377    0.227
size          -0.5437     0.7997  -0.680    0.527
CatVar1       -2.0183     1.0573  -1.909    0.115
size:CatVar1  -0.5047     1.2368  -0.408    0.700


To get the intercept for the CatVar1=1 line, one needs 0.7988-(-2.0183) = 2.18.  Is the standard error for this estimate 1.0573?


2) Suppose there were three or more categorical variables.  How can one perform pairwise comparisons between coefficients AND get the associated SE of the difference?  It seems that this would be quite desirable, yet I cannot find a package for this. I am looking for a method similar to the SAS implementation of the ESTIMATE statement.



-- 
Christopher M. Swan, Ph.D.
Associate Professor
Dept. of Geography & Environmental Systems
University of Maryland, Baltimore County
211 Sondheim Hall
1000 Hilltop Circle
Baltimore, MD 21250
Chris.Swan at umbc.edu
http://www.umbc.edu/people/cmswan
(410) 455-3957









On Sep 24, 2011, at 4:52 PM, Aitor Gastón wrote:

> 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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> 
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