Stepwise analysis with fixed variables

Thank you for the quick reply.

Two more questions:
1. For example, if this is my code:

RegModel =

lm(glucose~sex+BMI+height+weight+education+ses,weight=w_without_non_response)

summary(RegModel)
step(RegModel, direction ="backward",scope=list(lower=?,upper=?))

and I want the sex and height variables to be fixed, but the rest of the
variables to go into the backward analysis, how should I write the scope
function?

2.How can I add an alpha level to the step function as a criterion for the
backward regression analysis?

Thank you very much,

Einat



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