Re: Centering (was Re: Missing covariates)

From: "diane r mould" <drmould@attglobal.net> Subject: Re: Centering (was Re: Missing covariates) Date: Thu, 5 Jul 2001 12:07:32 -0400 Bill I agree with your statement about using common sense to pick a value. In a later email to NMUSERS, I clarified my position by saying: "The purpose of centering is two-fold. One purpose is to help out numerically and if the choice of value used for centering is far from the median or mean value for that demographic covariate, then you will certainly lose that benefit of centering. So if your study is in pediatric patients and you chose 65 yrs as the value to center age with, that could cause problems such as causing parameter values to become negative, or making it difficult to achieve convergence. If the latter occurred or could not be overcome by appropriate parameterization, one might be inclined to disregard potential covariates. So to that extent, I would agree that if you pick a value for centering that is greatly different from your present study you could have problems with the modeling exercise." So as always with modeling, I would summarize by saying that we should use common sense as the final rational for doing anything. :-) diane