Re: [R] scope argument in step function

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Sat 02 Jul 2005 - 16:41:44 EST

> What I wondered is if I pass the 'upper=~.' ,
> it seems step() thinks the full model is current one. Not adding
> anymore. If this is the right answer, is there a better way than
> creating fmla argument in the above?

Yes, that is exactly what the help page for step says it means. (So why are you unsure and asking?)

upper = terms(Response ~ ., data=ds3))

would appear to be a simpler way to get your intention.

On Fri, 1 Jul 2005, Young Cho wrote:

> Thanks a lot for help in advance. I am switching from matlab to R and I
> guess I need some time to get rolling. I was wondering why this code :
>
>> fit.0 <- lm( Response ~ 1, data = ds3)
>> step(fit.0,scope=list(upper=~.,lower=~1),data=ds3)
> Start: AIC= -32.66
> Response ~ 1
>
> Call:
> lm(formula = Response ~ 1, data = ds3)
> Coefficients:
> (Intercept)
> 1.301
>
>
> is not working different from the following:
>
>>
>> cnames <- dimnames(ds3)[[2]]
>> cnames <- cnames[-444] # last col is Response
>>
>> fmla <- as.formula(paste(" ~ ",paste(cnames,collapse="+")))
>> step(fit.0,scope=list(upper=fmla,lower=~1),data=ds3)
> Start: AIC= -32.66
> Response ~ 1
>> fmla <- as.formula(paste(" ~ ",paste(cnames,collapse="+")))
>> fit.s <- step(fit.0,scope=list(upper=fmla,lower=~1),data=ds3)
>
> Step: AIC= -Inf
> Response ~ ENTP9324 + CH1W0281
> Df Sum of Sq RSS AIC
> <none> 0 -Inf
> - CH1W0281 3 0.00381 0.00381 -115
> - ENTP9324 9 1 1 -34
>
> The dataframe ds3 is 17 by 444 and I understand it is not smart thing to
> run stepwise regression. What I wondered is if I pass the 'upper=~.' ,
> it seems step() thinks the full model is current one. Not adding
> anymore. If this is the right answer, is there a better way than
> creating fmla argument in the above?

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Received on Sat Jul 02 16:45:57 2005

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