[Rd] problem with zero-weighted observations in predict.lm?

[Peter Dalgaard]

Peter Dalgaard wrote:
> William Dunlap wrote:
>> In modelling functions some people like to use
>> a weight of 0 to drop an observation instead of
>> using a subset value of FALSE.  E.g.,
>>   weights=c(0,1,1,...)
>> instead of
>>   subset=c(FALSE, TRUE, TRUE, ...)
>> to drop the first observation.
>>
>> lm() and summary.lm() appear to treat these in the
>> same way, decrementing the number of degrees of
>> freedom for each dropped observation.  However,
>> predict.lm() does not treat them the same.  It
>> doesn't seem to diminish the df to account for the
>> 0-weighted observations.   E.g., the last printout
>> from the following script is as follows, where
>> predw is the prediction from the fit that used
>> 0-weights and preds is from using FALSE's in the
>> subset argument.  Is this difference proper?
> 
> Nice catch.
> 
> The issue is that the subset fit and the zero-weighted fit are not
> completely the same. Notice that the residuals vector has different
> length in the two analyses. With a simplified setup:
> 
>> length(lm(y~1,weights=w)$residuals)
> [1] 10
>> length(lm(y~1,subset=-1)$residuals)
> [1] 9
>> w
>  [1] 0 1 1 1 1 1 1 1 1 1
> 
> This in turn is what confuses predict.lm because it gets n and residual
> df from length(object$residuals). summary.lm() uses NROW(Qr$qr), and I
> suppose that predict.lm should follow suit.
> 

...and then when I went to fix it, I found that the actual line in the
sources (stats/R/lm.R) reads

 27442     ripley     n <- length(object$residuals) # NROW(object$qr$qr)

so it's been like that since December 2003. I wonder if Brian remembers
what the point was? (27442 was the restructuring into the stats package,
so it might not actually be Brian's code).

-pd


-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com