Hello- Such a strange problem, can't figure it out at all. Using binomial glm models, and the step() function, so the call looks like this:
sectionmodel = glm(formula = Target3 ~ S1Q12_NUM.1 + S1Q9_NUM.1 + S1Q5_NUM.1 + S1Q7_NUM.1 + S1Q8_NUM.1 + S1Q6_NUM.1 + S1Q10_NUM.1 + S1Q12_BURG.1 + S1Q12_CD.1 + S1Q4.1 + S1Q12_OTHVIOL.1 + S1Q8.1 + S1Q12_GBH.1 + S1Q11.1 + S1Q7.1 + S1Q12_THEFT.1 + S1Q12_DRIV.1 + S1Q5.1 + S1Q9.1 + S1Q12_DRUG.1, family = binomial, data = moddata) But when I run step() on the resulting model, some of the coefficents are doubled when it comes back, with a "2" at the end, e.g. like this: mymodel = step(sectionmodel, direction="backward", test="F") summary(mymodel) returns this: Coefficients: Estimate Std. Error z value Pr(>|z|) (Intercept) -4.58519 0.55675 -8.236 <2e-16 *** S1Q12_NUM.1 0.18446 0.08576 2.151 0.0315 * S1Q4.12 0.56893 0.40281 1.412 0.1578 S1Q12_OTHVIOL.11 0.56435 0.38262 1.475 0.1402 S1Q12_GBH.11 0.49199 0.33175 1.483 0.1381 S1Q7.11 -1.27330 1.12897 -1.128 0.2594 S1Q7.12 -1.83927 1.16909 -1.573 0.1157 S1Q5.11 0.91742 1.19489 0.768 0.4426 S1Q5.12 2.16861 1.19864 1.809 0.0704 . S1Q12_DRUG.11 -0.48400 0.29898 -1.619 0.1055 As you can see S1Q7.1 and S1Q5.1 are duplicated as "S1Q7.11" and "S1Q7.12" etc. I've googled and read and re-read the step() and stepAIC() documentation and I just can't figure out what it could mean. Removing the test="F" bit also generates the same behaviour. Any help greatly appreciated. Chris Beeley Institute of Mental Health, UK ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.