Bias in parameter estimates due to omitting gene-environment interaction terms in case-control studies.

Genetic studies are continuing to generate volumes and variety of data that can be used to examine the genetic effects. Often the effect of a genetic variant varies by nongenetic measures, what is traditionally defined as gene-environment interaction (G×E). If the G×E term is neglected, estimates of the main effects can be substantially biased. We derive a general and convenient approximation to the magnitude of bias in the estimates due to omitting the G×E term. We show that the approximation is reasonably accurate in finite samples. We then apply the approximation in a study of Alzheimer's disease.