Dear All, I am attempting to use forward and/or backward selection to determine the best model for the variables I have. Unfortunately, because I am dealing with patients and every patient is receiving treatment I need to force the variable for treatment into the model. Is there a way to do this using R? (Additionally, the model is stratified by randomisation period). I know that SAS can be used to do this but my SAS coding is poor and consequently it would be easier for me to use R, especially given the fractional polynomial transformations! Currently the model is as follows (without treatment). coxfita=coxph(Surv(rem.Remtime,rem.Rcens)~sind(nearma)+fsh(nearma)+fdr(nearma)+th1(nearma)+th2(nearma)+fp(cage)+fp(fint)+fp(tsb)+strata(rpa),data=nearma) Thank you for your help, Laura
Forcing a variableinto a model using stepAIC
2 messages · Laura Bonnett, Chuck Cleland
On 5/22/2009 9:58 AM, Laura Bonnett wrote:
Dear All, I am attempting to use forward and/or backward selection to determine the best model for the variables I have. Unfortunately, because I am dealing with patients and every patient is receiving treatment I need to force the variable for treatment into the model. Is there a way to do this using R? (Additionally, the model is stratified by randomisation period). I know that SAS can be used to do this but my SAS coding is poor and consequently it would be easier for me to use R, especially given the fractional polynomial transformations! Currently the model is as follows (without treatment). coxfita=coxph(Surv(rem.Remtime,rem.Rcens)~sind(nearma)+fsh(nearma)+fdr(nearma)+th1(nearma)+th2(nearma)+fp(cage)+fp(fint)+fp(tsb)+strata(rpa),data=nearma) Thank you for your help, Laura
See the scope argument to stepAIC in the MASS package. You can specify a formula in the 'lower' component of scope which includes the treatment variable. That will force the treatment variable to remain in every model examined in the stepwise search.
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Chuck Cleland, Ph.D. NDRI, Inc. (www.ndri.org) 71 West 23rd Street, 8th floor New York, NY 10010 tel: (212) 845-4495 (Tu, Th) tel: (732) 512-0171 (M, W, F) fax: (917) 438-0894