Correlations between estimated and true dietary intakes.

PURPOSE: It is unclear how well questionnaire or so-called reference methods of dietary assessment correlate with true dietary intake. We develop a method to estimate such correlations.
METHODS: An error model is described that uses data from a food frequency questionnaire (Q), a reference method (R), and a biological marker (M). The model does not assume the classical error model for either R or M, or that the correlation between errors in the questionnaire and reference data is zero. Credible intervals can be placed about correlations between R, Q, M and true dietary data (T), also about the correlations between errors in reference and questionnaire data.
RESULTS: Application of this model to a validation data set mainly found correlations in the range 0.4 to 0.8, and that correlations (R,T) generally exceeded correlations (Q,T), providing evidence that R is more valid than Q. Estimated correlations between errors in R and Q were often far from zero suggesting that regression calibration to imperfect reference data is problematic unless these error correlations can be estimated.
CONCLUSION: A biological marker in addition to dietary data, allows calculation of correlations between estimated and true dietary intakes under reasonable assumptions about errors. However, sensitivity analyses are necessary on one variable.