RE: When to do transformation of data?

From: Leonid Gibiansky Subject: RE: [NMusers] When to do transformation of data? Date: Tue, 23 Apr 2002 11:21:59 -0400 Chuanpu I thought about something like the model ln(Y)=ln(F+EPS1) + EPS2 then Y=F EPS(2) + EPS1*EXP(EPS2) I checked that for SD(EPS2) < 0.2 the distribution of EPS1*EXP(EPS2) is almost normal. For SD(EPS2) = 0.3 the distribution of EPS1*EXP(EPS2) is very close to normal. For SD(EPS2) = 0.5 the distribution of EPS1*EXP(EPS2) differs from normal on the tails: deletion of 0.5% of the highest and 0.5% of the lowest values makes it sufficiently similar to normal. I have not tried this approach, but this might be useful in some cases, at least to test the presence of additive errors. So what would you say about NONMEM code ( Y=LOG(DV) ) : F1=F+EPS1 IF(F1 < 0.001) F1= 0.001 Y=LOG(F1)+EPS2 Will it work and will it be reasonable? Leonid