Accurately quantifying environmental exposures at the individual level is a challenge that is widely recognized and the topic of the NIH Genes, Environment and Health Initiative. The difficulty of this task, however, may be most evident when contrasted with the detail and volume of information obtained from genetic samples. In all three data sets, measures of environmental variables were comparatively crude, if available at all. Although the FHS data includes a variable for cigarettes smoked per day at each visit, missing data and the lack of information about smoking before and between visits make it difficult to quantify smoking behavior. Given the structure of smoking data, a choice has to be made between creating crude categories of exposure (e.g., ever or never smoked) and basing the ascertainment of exposure on a limited time period (e.g., average number of cigarettes smoked 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
