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logistic regression having problems with categorical variables

From: dr soumalya ray
Subject: logistic regression having problems with categorical variables
Date: Sat, 19 Dec 2015 06:28:49 +0000 (UTC)

hello friends,

i'd like to point out a problem present in the "logistic regression" function 
of PSPP.

this is a problem stopping usage of PSPP for logistic regression analysis. 

short description: while doing the logistic regression, pspp completely ignores 
"measure" of the variable. it is treating categorical variable as a continuous 
variable leading to wrong results.

complete description:
1) for reporting this, i am using the example given in this SPSS tutorial-
2) look at the example dataset - binary.sav
3) open the file.
4) it contains four variables. two nominal (admit and rank) and two scale (gre 
and gpa)[visible in the "measure" of variable view].
5)go to: analyse -> regression-> logistic regression
6)select admit as the dependent variable and rank,gre and gpa as the 
independent variable. 

expected result: the output should look like this -

actual result in pspp:

7) possible reason: pspp ignores the measure indicator of the variable. it 
treats rank as a continuous variable. 

8) this is a long term problem and it exists since the function is available in 
pspp. did not report it as was unable to confirm the expected result from SPSS. 

9) interestingly, if this file is imported in R and logistic regression 
analysis done, expected result takes place. 

pspp version: psppire 0.8.5 (problem present since logistic regression function 
added in PSPP)
uname -a:Linux som 4.2.0-21-generic #25~14.04.1-Ubuntu SMP Thu Dec 3 13:55:58 
UTC 2015 x86_64 x86_64 x86_64 GNU/Linux



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