Re: [R] linear model and by()

[David Winsemius]

On Nov 12, 2009, at 8:26 PM, Sam Albers 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.

Beacuase the results are a layer deeper than summary is describing;
Try:
lapply(lm.r, summary)

(this theory tested with one of the standard datasets, Indometh:

library(utils)
data(Indometh)
reglist <- by(Indometh, Indometh$Subject, function(x) lm(conc~time,  
data=x) )
> summary(reglist)
  Length Class Mode
1 12     lm    list
4 12     lm    list
2 12     lm    list
5 12     lm    list
6 12     lm    list
3 12     lm    list

Whereas, without the assignment to reglist I got a series of expected  
lm output, so by "works".

Similarly, although I wasn't hopeful that this would work, when I  
tried to
examine the residuals, I get these warning messages:

similarly:

lapply(lm.r, plot)


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

Sorry to not use your data but it's not in a form that lends itself  
very well to quick testing. If you had included the input commands I  
might have tried it.

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

David Winsemius, MD
Heritage Laboratories
West Hartford, CT

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