Residuals and INTERACTION for simultaneous PK/PD, impossible?

From: "Eleveld, DJ" d.j.eleveld@anest.umcg.nl Subject: [NMusers] Residuals and INTERACTION for simultaneous PK/PD, impossible? Date: Fri, April 15, 2005 7:34 am Hi everyone, Thanks very much to the very knowledgeable users of the list I understand that INTERACTION should be used when residuals are expected to be heteroscastic and shouldnt be used when residuals are expected to be homoscastic. However I am confused as what to do in the simultaneous PK/PD case. Here my PK residuals are expected to be heteroscastic (so INTEACTION should be used) but my PD residuals are expected to be homoscastic (so INTERACTION should not be used). I cant do both at the same time, but my data does contain both kinds of residuals at the same time. There is much discussion in the archives about simultaneious PK/PD but they seem to discuss finding the 'right' parameters. I could find no posts that brought up if INTERACTION should be used or not. I seem to get better fitting without INTERACTION, but I think this is simply just due to the fact that I have more PD data then PK data. Anyone have any ideas about the correct way to approach this? Thanks very much, Doug Eleveld