[R] Stepwise analysis with fixed variables

[Einat]

These are my 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 :) 



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