There is additional uncertainty in environmental data stemming from our lack of knowledge of the timing of effects. For instance, even if we could measure a person’s pack years of smoking without error for a study of breast cancer risk, we might still need to know whether smoking was initiated prior to her first birth, and how long ago she quit smoking. These considerations are compounded for studies of the impact of in utero and early childhood exposures on a disease with late onset, and highlight the need for methods and designs, such as longitudinal studies, that consider interactions over time and time-varying exposures.
