Dear all,
A friend of mine who is not subscribed has a doubt concerning "weight and
logistic regression". He asked me to submit his question.
Many thanks in advance,
Rodrigo Pereira
________________________
Hi,
I have a problem on the usage of S-plus to carry a logistic regression that
I believe someone in the list could help.
I have data on whether a stone in a streambed moved in relation to three
independent variables:
(1) amount of rainfall in the 2-months period stones remained in the
stream, treated as continuos, 6 periods.
(2) mean discharge of the stream, treated as continuos, 5 streams.
(3) size of the stone, categorical, 2 size classes.
I have thus 60 (=6 x 5 x 2) combinations of the variables. For each
combination, I have data on exactly 15 experimental, previously painted stones.
A critic raised by a referee was that stones in the 15 stones set were not
independent to each other. If a stone in a given combination of treatments
moved, it is likely that other would move too as they are neighborhoods
spatially. They are pseudoreplications. In other words, I could not enter
each stone as an observation (899 d.f. ~ 60 x 15).
I believe the referee should be right. After studying some books and the
S-plus help, I found that there is a possibility to enter data as
proportions and then assigning a weight to it. In my case, I would use each
15 stones set to produce a proportion. I preliminarily carried the analysis
using proportions and observed that S-plus treats each proportion as one
degree of freedom (59 d.f. for the study). However, given that each
proportion is based on exactly 15 stones, each proportion has exactly the
same weight. I carried some analysis using in each one weights 1, 5 and 10
and observed that each one produced different results (estimates are the
same, but error of them as well as deviance are different).
My questions are:
Do someone know what exactly WEIGHT do?
How should I weight my data?
Am I doing the correct analysis?
Any comment will be of great value.
Best wishes,
Adriano S. Melo <grumicha@yahoo.com.br>
Dep. Biologia, FFCLRP
Universidade de São Paulo
14040-901 - Ribeirão Preto - SP
Brazil
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