Validation of the exposure assessment approach developed will be a critical part of the proposal. The committee recognizes that retrospective exposure assessment of herbicide exposures among Vietnam War veterans may, at best, be semiquantitative or rank ordered. Because we have a generally poor understanding of the sources and magnitudes of exposures to herbicides that occurred in Vietnam more than 25 years ago, the predicted levels of exposure to individual veterans are likely to be imprecise. Thus, applicants should discuss the anticipated sources and magnitudes of uncertainty in their model predictions. Attention should also be paid to the role that quantitative or semiquantitative data might play in the validation of exposure predictions in a pilot study.

The best method for validation is not obvious and is left to the researchers to propose, critique, and defend. The limitations and uncertainties associated with the validation method—such as those discussed previously for serum TCDD levels—should be addressed.

Respondents are encouraged to consider other alternative approaches to the validation process.