RE: model for OMEGA and SIGMA

From: "Bachman, William" Subject:RE: [NMusers] model for OMEGA and SIGMA Date: Fri, 7 Feb 2003 13:25:38 -0500 Luciane, Basically you can code anything you want (you're not limited to additive, proportional, exponential, etc.). But, the idea is that your error structure reflect your data! So, typically we use a proportional or exponetial inter-individual error model because PK parameters like V & CL are often log-normally distributed. By the same token, if you know something about the residual error distribution, e.g. from the characteristics of your assay, etc, you can make some assumptions about what the residual error model should be. As an example, you might have an assay where the error is proportional over most of the range of concentrations but constant near the limits of detection. In that case, and additive plus proportional residual error model might be an appropriate choice: Y = F + F*ERR(1) + ERR(2) Finally, fit your model to your data and test your assumptions. Bill