[R] Error when using step

[David Winsemius]

On Jun 22, 2009, at 4:08 AM, Dieter Menne wrote:

> Chris Friedl <cfriedalek <at> gmail.com> writes:
>
>> I have two questions about the built-in function step. Ultimately I  
>> want to
>> apply a lm fitting and subsequent step procedure to thousands of  
>> data sets
>> groups by a factor defined as a unique ID.
>>
>> Q1. The code below creates a data.frame comprising three marginally  
>> noisy
>> surfaces. The code below fails when I use step in a function but  
>> summary
>> seems to show the model fits are legitimate. Any ideas on what I'm  
>> doing
>> wrong?
>
> ... Well designed example code omitted
>
>> function(x){lm(model, data = x)})
>> lapply(fits, function(x){summary(x)})
>>
>> # FAIL
>> lapply(fits, function(x){step(x)})
> ....
>> Error in as.data.frame.default(data) :
>>  cannot coerce class "lm" into a data.frame
>
> Looks like an environment problem. I could not find a workaround  
> quickly,

Perhaps:

by(data2, data2$grp, function(x) step(lm(model, data=x)))


> but you might have a look at
>
> http://finzi.psych.upenn.edu/R/Rhelp02a/archive/16599.html
>
> We call it "Ripley's Game" here, because variants of it can help you  
> quite often.
>
> Dieter
>


David Winsemius, MD
Heritage Laboratories
West Hartford, CT