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Date:         Mon, 23 Apr 2007 09:45:46 +0200
Reply-To:     Marta García-Granero
              <biostatistics@terra.es>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Marta García-Granero
              <biostatistics@terra.es>
Organization: Asesoría Bioestadística
Subject:      Re: Poisson log-linear regression
In-Reply-To:  <200704121729.l3CFcmIa008670@mailgw.cc.uga.edu>
Content-Type: text/plain; charset=ISO-8859-15

Hi Ken

Since I went a bit "delete-happy" with the messages (I had over 400 stored I hadn't looked at), I don't know if you got a reply to your message dated 12 april.

Here it goes mine:

DATA LIST FREE /id(F8) age(F8) matings(F8). BEGIN DATA. 1 27 0 2 28 1 3 28 1 4 28 1 5 28 3 6 29 0 7 29 0 8 29 0 9 29 2 10 29 2 11 29 2 12 30 1 13 32 2 14 33 4 15 33 3 16 33 3 17 33 3 18 33 2 19 34 1 20 34 1 21 34 2 22 34 3 23 36 5 24 36 6 25 37 1 26 37 1 27 37 6 28 38 2 29 39 1 30 41 3 31 42 4 32 43 0 33 43 2 34 43 3 35 43 4 36 43 9 37 44 3 38 45 5 39 47 7 40 48 2 41 52 9 END DATA.

RECODE matings (0=1E-06) . WEIGHT BY matings . GENLOG id WITH age /MODEL = POISSON /PRINT = ESTIM /PLOT = NONE /DESIGN age .

As you can see, you have to give a very low weight to 0 frequencies (replace them by 1E-06) and then weight your dataset by MATINGS.

The result GENLOG gives is the same as the textbook.

KC> I am trying to work on this dataset (see below) using Poisson Regression.

KC> According to the textbook (Ramsey, The Statistical Sleuth 2nd ed), the KC> esimates should be:

KC> intercept -1.582 KC> age 0.0687

KC> I did get the same result with SAS proc genmod. However, I cannot obtain KC> the same result with General log linear model in SPSS.

KC> I specify the ID as factor, age as covariate, and matings as cell structure. KC> In the model I only put in the main effect of age. The result wasn't even KC> close (age actually shows negative estimate).

-- Regards, Dr. Marta García-Granero,PhD mailto:biostatistics@terra.es Statistician

--- "It is unwise to use a statistical procedure whose use one does not understand. SPSS syntax guide cannot supply this knowledge, and it is certainly no substitute for the basic understanding of statistics and statistical thinking that is essential for the wise choice of methods and the correct interpretation of their results".

(Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)


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