The food frequency questionnaire.

To the Editors: Of course we agree with Kristal et al. (1) that validity is an issue of degree rather than a dichotomy and that multiple methods will usually be useful when evaluating validity because a perfect standard is rarely available. However, their examples based on face validity, construct validity, and predictive validity do not provide evidence that food frequency questionnaires should be abandoned. Face validity may have some value when data are not available, but their example based on a single item from our questionnaire (frequency of consuming ''beef, pork, or lamb as a sandwich or mixed dish, such as, stew, casserole, lasagna, etc.'') shows how this subjective assessment can be abused. Certainly, we would never draw any conclusions from this question alone. However, when used in combination with our main meat question (frequency of consumption of beef, pork, or lamb as a main dish, such as a steak or roast) and four other questions on red meat, this question helps identify individuals with very different intakes of red meat and, in conjunction with other questions, very different intakes of saturated fat and cholesterol. The item on meat in a sandwich or mixed dish (which we consider a ''cleaner question'') reduces the inclusion of persons who rarely use beef, pork, or lamb as a main dish in a low red meat category if they frequently use these foods in other circumstances. Contrary to their implications , we do not use this question to estimate intakes of vegetables, cheese, or fats in food preparation, which are assessed by other items. Kristal et al. argue that the face validity of actual intakes as assessed by diet records is far greater. However, highly precise details about the constituents of a mixed dish are not useful if this dish is eaten only occasionally or perhaps never again. Thus, the value of this information as a measure of long-term diet is questionable. If many days of diet records are collected, we agree that this would be a superior method of dietary assessment, but even carefully weighed 7-day diet records provide intraclass correlations on the order of 0.6 for nutrient intakes (2), indicating substantial error from within-person variation alone. Surprisingly, Kristal et al. ignore the use of weighed diet records for assessing construct validity as they argue that these provide a superior method of dietary assessment. Although they are not perfect for this purpose, with …