[R] inconsistent behaviour of add1 and drop1 with a weighted linear model

[Jenny Hodgson]

I was using Version 2.3.1 for Windows (binary download version). Guess I
should be using the latest version, (but the reason is I'm writing up my
PhD and I thought my results would be more 'repeatable' if I didn't keep
changing my version of the software, I didn't really think there would
be any glitches as big as this). Sorry if this is a waste of your time.
And thanks very much for replying so quickly.

Jenny

_____________
Jenny Hodgson
Department of Biology - area 18
PO box 373
University of York
YO10 5YW
UK
Tel: 01904 328623



Peter Dalgaard wrote:

> Jenny Hodgson wrote:
>
>> Dear R Help,
>> I have noticed some inconsistent behaviour of add1 and drop1 with a 
>> weighted linear model, which affects the interpretation of the results.
>> I have these data to fit with a linear model, I want to weight them 
>> by the relative size of the geographical areas they represent.
>> _________________________________________________________________________________________ 
>>
>>  > example
>>            y        x1       x2   weights
>> 1  -4.546477 0.1859556 50.00000 0.9466022
>> 2   1.484246 0.4740497 29.88000 1.3252430
>> 3   2.203681 0.8594264 16.93333 0.9466022
>> 4   1.943163 0.8713360 42.11765 2.7766997
>> 5   1.886473 0.9006082 19.00000 0.9466022
>> 6   1.795393 0.8455183 23.68421 1.1674760
>> 7   1.878077 0.5641396 35.00000 0.8203885
>> 8  -4.215484 0.4356858 58.75000 0.4417477
>> 9   1.993339 0.5440061 19.28571 0.8519420
>> 10  1.560869 0.6285066 19.54545 0.8203885
>> 11  2.761535 0.7017720 15.83333 0.1262136
>> 12  0.995959 0.4638751  0.00000 0.9466022
>> 13 -4.516514 0.2794199 77.85714 0.8834954
>>  > sum(example$weights)
>> [1] 13.00000
>>
>>  > model<-lm(y~1,data=example,weights=weights)
>>  > add1(model,.~.+x1+x2)
>> Single term additions
>>
>> Model:
>> y ~ 1
>>        Df Sum of Sq    RSS    AIC
>> <none>              94.000 27.719
>> x1      1    55.290 38.710 18.185
>> x2      1    58.630 35.371 17.012
>>   
>
> Which version of R??!
>
> I get (2.4.1 on Fedora 6):
>
> > add1(model,.~.+x1+x2)
> Single term additions
>
> Model:
> y ~ 1
>       Df Sum of Sq    RSS    AIC
> <none>              74.541 24.703
> x1      1    44.377 30.164 14.942
> x2      1    36.619 37.922 17.918
>