[R] Adding a subset to a glm messes up factors?

[Prof Brian Ripley]

First, 'subset' is an argument to glm(), but for some reason you did not 
use it.  Your subject line is quite misleading, and had it been the more 
accurate

 	Adding a 'data' argument to glm messes up factors?

you might have realised the problem.

Second, your models are fitted to different datasets: the first to objects 
in your workspace, and the second to columns of data.all. Since you have 
not (as we asked) given a reproducible example we cannot know what those 
differences are, but differences in the datasets will be the key.

Third, the best way to fit linear models is lm(), not 
glm(family=gaussian).


On Fri, 7 Dec 2007, Muri Soares wrote:

> I have a problem with running a glm using a subset of my data. Whenever 
> I choose a subset, in the summary the factors arent shown (as if the 
> variable was a continuous variable). If I dont use subsets then all the 
> factors are shown. I have copied the output from summary for both cases.
>
> Thanks for the help,
> Muri
>
>> model<-glm(log(cpue)~year,family=gaussian)
> Call:
> glm(formula = log(cpue) ~ year, family = gaussian)
>
> Deviance Residuals:
>    Min       1Q   Median       3Q      Max
> -2.0962  -0.5851  -0.1241   0.4805   3.9236
>
> Coefficients:
>            Estimate Std. Error t value Pr(>|t|)
> (Intercept)   0.8899     0.1844   4.825 1.42e-06 ***
> year1990     -0.6107     0.1925  -3.173  0.00152 **
> year1991     -1.7466     0.1902  -9.184  < 2e-16 ***
> year1992     -1.4061     0.1864  -7.544 5.07e-14 ***
> year1993     -1.4069     0.1860  -7.565 4.31e-14 ***
> ...
>
>> model<-glm(log(cpue)~year,family=gaussian,subset(data.all,species=="n")
> Call:
> glm(formula = log(cpue) ~ year, family = gaussian, data = subset(data.all,
>    species == "n"))
>
> Deviance Residuals:
>     Min        1Q    Median        3Q       Max
> -1.64577  -0.61671  -0.08972   0.55792   2.73737
>
> Coefficients:
>             Estimate Std. Error t value Pr(>|t|)
> (Intercept) 32.446570  10.076895   3.220  0.00135 **
> year        -0.016345   0.005037  -3.245  0.00123 **
> ---

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>
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PLEASE do!

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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