Stepwise analysis with fixed variables

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Einat
Sent: Wednesday, November 21, 2012 1:41 PM
To: r-help at r-project.org
Subject: Re: [R] Stepwise analysis with fixed variables

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_res
ponse)

  >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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