Issues in reproducibility and validity of dietary studies.

Numerous factors affect the reproducibility and validity of dietary assessment questionnaires. Although the respondents' abilities to respond accurately are most frequently discussed as the cause of apparently poor reproducibility and validity, many other factors are as important and perhaps more important. Most of these other factors are under the control of the investigator, and thus are amenable to improvement. Factors which may affect reproducibility included the degree of variability permitted by the instrument, the error-proneness of the response format, quality control of coding and keying and real dietary change in the time between the two administrations of the questionnaire. Factors affecting real or apparent validity include respondent characteristics, questionnaire design and quantification, quality control, and the adequacy of the reference data. The implications of inadequate reference data are illustrated and discussed.