[R] Some coefficients are doubled when I use the step() function

[Chris Beeley]

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