RE: covariate significance

From: "Piotrovskij, Vladimir [PRDBE]" Subject: RE: [NMusers] covariate significance Date: Sun, 21 Apr 2002 10:55:16 +0200 Jorn, Your COV may represent levels of a categorical covariate (factor). E.g. COV is race and 0, 1 and 2 code the three basic races. In that case your choice 1 is the only one (if you have sufficient evidences to assume that COV=1 and COV=2 produce equal effect of X). Alternatively, COV may be a quantitative variable, and 0, 1 and 2 are values (e.g., doses), not levels. Then, only choice 2 is applicable. You could write your expression, which is in fact a linear model, as follows: X = THETA(1)*(1+THETA(2)*COV) thereby avoiding the IF ... ELSE loop. The parameters are better tractable: THETA(1) is an intercept and THETA(2) is a slope expressed in terms of the fractional change. In any case you cannot select one of the two implementations based on goodness-of-fit or other statistical criteria. Best regards, Vladimir