ESTIMATING THE ACCURACY OF DIETARY QUESTIONNAIRE ASSESSMENTS - VALIDATION IN TERMS OF STRUCTURAL EQUATION MODELS

The validity and precision of questionnaire assessments of the habitua l intake of individuals are usually evaluated by comparison with refer ence measurements that are supposed to provide a best possible substit ute for the individuals' true intake values. In the present paper, a m easurement error model is presented, defining different types of error - random or systematic, and within or between individuals - that may occur in dietary intake measurements. It is then discussed how simple latent variable models (structural equation models) can be used to est imate the average magnitude of these various types of error. So far, a pproaches described for the analysis of dietary validity studies have all been based on the assumption that the random errors of repeat refe rence measurements, taken by the same method on different occasions, a re uncorrelated, so that the average of a sufficiently large number of repeat reference measurements will provide an accurate ranking of ind ividuals by true intake level. In the present paper it is described ho w, by additional comparison with a third type of measurement such as a biochemical marker, the validity of dietary questionnaire measurement s can be evaluated even in situations where the random errors of repea t reference measurements can no longer be assumed to be independent.