[R] linear model and by()

Ista Zahn istazahn at gmail.com
Fri Nov 13 03:01:52 CET 2009 

Hi,
You have not given us all the data needed to reproduce your analysis
(what is SectionF?), but the issue is probably that lm.r is a list and
you're not treating it that way. Try

srt(lm.r)

and

summary(lm.r[[1]])

You may also want to look at the the lmList() function in the lme4 package.

-Ista

On Thu, Nov 12, 2009 at 8:26 PM, Sam Albers <tonightsthenight at gmail.com> wrote:
> Hello R list,
>
> This is a question for anyone who has used the by() command. I would like to
> perform a regression on a data frame by several factors. Using by() I think
> that I have able to perform this using the following:
>
>> lm.r <- by(master, list(Sectionf=Sectionf, startd=startd), function(x) lm
> (tot.c ~ starttime, data = x))
>
> So that is, I would like to perform separate regressions for each level of
> Sectionf for each level of startd. Now I can get the coefficients and
> intercepts from all the fitted models. However, I am now unsure how to glean
> more information from the regressions. If I follow up with this command I
> get this:
>
>> summary(lm.r)
>  9-09-04.Length  9-09-04.Class  9-09-04.Mode
>  12    lm    list
>  12    lm    list
>  12    lm    list
>  9-09-11.Length  9-09-11.Class  9-09-11.Mode
>  12    lm    list
>  12    lm    list
>  12    lm    list
>
> Under normal circumstances, this usually ends up giving me more info about
> the fitted model, but some reason when using the by() command it doesn't.
> Similarly, although I wasn't hopeful that this would work, when I tried to
> examine the residuals, I get these warning messages:
>
>> plot(lm.r$fit,lm.r$res,xlab="Fitted",ylab="Residuals")
> Error in plot.window(...) : need finite 'xlim' values
> In addition: Warning messages:
> 1: In min(x) : no non-missing arguments to min; returning Inf
> 2: In max(x) : no non-missing arguments to max; returning -Inf
> 3: In min(x) : no non-missing arguments to min; returning Inf
> 4: In max(x) : no non-missing arguments to max; returning -Inf
>
> So my question might be answered by someone saying that this isn't a good
> way to do this. Still, can anyone suggest some strategies that might help me
> out here? Specifically, how might I be able work with these regressions once
> I have fitted all the models. Obviously I need to examine the residuals,
> look at the model fit, etc. Any suggestions on what I might be doing wrong
> would be much appreciated. I am also including a subset of the data and the
> commands I used in case that is helpful.
>
> I am using Ubuntu 9.04 and R 2.8.1
>
> Thanks in advance!
>
> Sam
>
>> attach(master)
>> Sectionf <- factor(Section)
>> startd=as.Date(startdate, format="%d/%m/%Y")
>
>> lm.r <- by(master, list(Sectionf=Sectionf, startd=startd), function(x) lm
> (tot.c ~ starttime, data = x))
>
> Data:
>
> "Section","starttime","startdate","tot.c"
> "Upstream",0,04/09/09,0.17
> "Upstream",0,04/09/09,0.19
> "Upstream",0,04/09/09,0.14
> "Middle",0,04/09/09,0.2
> "Middle",0,04/09/09,0.13
> "Middle",0,04/09/09,0.11
> "Downstream",0,04/09/09,0.16
> "Downstream",0,04/09/09,0.17
> "Downstream",0,04/09/09,0.17
> "Upstream",25,04/09/09,0.17
> "Upstream",25,04/09/09,0.19
> "Upstream",25,04/09/09,0.14
> "Middle",25,04/09/09,0.2
> "Middle",25,04/09/09,0.13
> "Middle",25,04/09/09,0.11
> "Downstream",25,04/09/09,0.16
> "Downstream",25,04/09/09,0.17
> "Downstream",25,04/09/09,0.17
> "Upstream",50.06,04/09/09,0.17
> "Upstream",50.06,04/09/09,0.19
> "Upstream",50.06,04/09/09,0.14
> "Middle",50.06,04/09/09,0.2
> "Middle",50.06,04/09/09,0.13
> "Middle",50.06,04/09/09,0.11
> "Downstream",50.06,04/09/09,0.16
> "Downstream",50.06,04/09/09,0.17
> "Downstream",50.06,04/09/09,0.17
> "Upstream",75.42,04/09/09,0.17
> "Upstream",75.42,04/09/09,0.19
> "Upstream",75.42,04/09/09,0.14
> "Middle",75.42,04/09/09,0.2
> "Middle",75.42,04/09/09,0.13
> "Middle",75.42,04/09/09,0.11
> "Downstream",75.42,04/09/09,0.16
> "Downstream",75.42,04/09/09,0.17
> "Downstream",75.42,04/09/09,0.17
> "Upstream",100.14,04/09/09,0.17
> "Upstream",100.14,04/09/09,0.19
> "Upstream",100.14,04/09/09,0.14
> "Middle",100.14,04/09/09,0.2
> "Middle",100.14,04/09/09,0.13
> "Middle",100.14,04/09/09,0.11
> "Downstream",100.14,04/09/09,0.16
> "Downstream",100.14,04/09/09,0.17
> "Downstream",100.14,04/09/09,0.17
> "Upstream",125.31,04/09/09,0.17
> "Upstream",125.31,04/09/09,0.19
> "Upstream",125.31,04/09/09,0.14
> "Middle",125.31,04/09/09,0.2
> "Middle",125.31,04/09/09,0.13
> "Middle",125.31,04/09/09,0.11
> "Downstream",125.31,04/09/09,0.16
> "Downstream",125.31,04/09/09,0.17
> "Downstream",125.31,04/09/09,0.17
> "Upstream",150.29,04/09/09,0.17
> "Upstream",150.29,04/09/09,0.19
> "Upstream",150.29,04/09/09,0.14
> "Middle",150.29,04/09/09,0.2
> "Middle",150.29,04/09/09,0.13
> "Middle",150.29,04/09/09,0.11
> "Downstream",150.29,04/09/09,0.16
> "Downstream",150.29,04/09/09,0.17
> "Downstream",150.29,04/09/09,0.17
> "Upstream",0,11/09/09,0.12
> "Upstream",0,11/09/09,0.16
> "Upstream",0,11/09/09,0.12
> "Middle",0,11/09/09,0.08
> "Middle",0,11/09/09,0.12
> "Middle",0,11/09/09,0.1
> "Downstream",0,11/09/09,0.11
> "Downstream",0,11/09/09,0.13
> "Downstream",0,11/09/09,0.13
> "Upstream",25,11/09/09,0.12
> "Upstream",25,11/09/09,0.16
> "Upstream",25,11/09/09,0.12
> "Middle",25,11/09/09,0.08
> "Middle",25,11/09/09,0.12
> "Middle",25,11/09/09,0.1
> "Downstream",25,11/09/09,0.11
> "Downstream",25,11/09/09,0.13
> "Downstream",25,11/09/09,0.13
> "Upstream",50,11/09/09,0.12
> "Upstream",50,11/09/09,0.16
> "Upstream",50,11/09/09,0.12
> "Middle",50,11/09/09,0.08
> "Middle",50,11/09/09,0.12
> "Middle",50,11/09/09,0.1
> "Downstream",50,11/09/09,0.11
> "Downstream",50,11/09/09,0.13
> "Downstream",50,11/09/09,0.13
> "Upstream",75,11/09/09,0.12
> "Upstream",75,11/09/09,0.16
> "Upstream",75,11/09/09,0.12
> "Middle",75,11/09/09,0.08
> "Middle",75,11/09/09,0.12
> "Middle",75,11/09/09,0.1
> "Downstream",75,11/09/09,0.11
> "Downstream",75,11/09/09,0.13
> "Downstream",75,11/09/09,0.13
> "Upstream",100,11/09/09,0.12
> "Upstream",100,11/09/09,0.16
> "Upstream",100,11/09/09,0.12
> "Middle",100,11/09/09,0.08
> "Middle",100,11/09/09,0.12
> "Middle",100,11/09/09,0.1
> "Downstream",100,11/09/09,0.11
> "Downstream",100,11/09/09,0.13
> "Downstream",100,11/09/09,0.13
> "Upstream",125.04,11/09/09,0.12
> "Upstream",125.04,11/09/09,0.16
> "Upstream",125.04,11/09/09,0.12
> "Middle",125.04,11/09/09,0.08
> "Middle",125.04,11/09/09,0.12
> "Middle",125.04,11/09/09,0.1
> "Downstream",125.04,11/09/09,0.11
> "Downstream",125.04,11/09/09,0.13
> "Downstream",125.04,11/09/09,0.13
>
>
>
>
>
>
> --
> *****************************************************
> Sam Albers
> Geography Program
> University of Northern British Columbia
> 3333 University Way
> Prince George, British Columbia
> Canada, V2N 4Z9
> phone: 250 960-6777
> *****************************************************
>
>        [[alternative HTML version deleted]]
>
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>



-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org

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