Michele,
This error means that some of the variables in your formula have missing values,
and hence when these terms or added/dropped you have a different sample size.
Hence, the AIC cannot be compared between different models. The solution is to
create a compelete-data dataframe for the largest model, i.e none of the
variables in the largest model should have any missing values. Then run stepAIC
on this dataframe.
Best,
Ravi.
____________________________________________________________________
Ravi Varadhan, Ph.D.
Assistant Professor,
Division of Geriatric Medicine and Gerontology
School of Medicine
Johns Hopkins University
Ph. (410) 502-2619
email: rvaradhan at jhmi.edu
----- Original Message -----
From: Michele Santacatterina <miksanta at gmail.com>
Date: Wednesday, January 28, 2009 9:51 am
Subject: [R] StepAIC with coxph
To: R-help at r-project.org
> Hi,
>
> i'm trying to apply StepAIC with coxph...but i have the same error:
>
> stepAIC(fitBMT)
> Start: AIC=327.77
> Surv(TEMPO,morto==1) ? VOD + SESSO + ETA + ........
>
> Error in dropterm.default(fit,scope$drop, scale=scale,trace=max(0, :
> number of rows in use has changed: remove missing values?
>
> anybody know this error??
>
> Thanks.
>
> Michele
>
> [[alternative HTML version deleted]]
>
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