per day across visits, or last known smoking status). More complete measurements of environmental exposures across time would not only better represent the exposure of interest, but would also allow greater flexibility to replicate findings and to compare with other studies in which exposures are computed differently. Several groups selected sex as an “environmental” variable of interest, which could represent a proxy for different environmental exposures [Arya et al., 2009]. While the measurement of sex is certainly more straightforward than smoking or alcohol consumption, it does not necessarily provide more insight into the causal pathways of specific environmental exposures. Because many health outcomes are thought to be a complex combination of environmental and genetic factors, future studies should strive to create new methods for the collection of meaningful environmental information that is as reliable and comprehensive as the genotype data.
