Empirical evidence of correlated biases in dietary assessment instruments and its implications

Multiple-day food records or 24-hour recalls are currently used as "referen ce" instruments to calibrate food frequency questionnaires (FFQs) and to ad just findings from nutritional epidemiologic studies for measurement error. The common adjustment is based on the critical requirements that errors in the reference instrument be independent of those in the FFQ and of true in take. When data on urinary nitrogen level, a valid reference biomarker for nitrogen intake, are used, evidence suggests that a dietary report referenc e instrument does not meet these requirements. In this paper, the authors i ntroduce a new model that includes, for both the FFQ and the dietary report reference instrument, group-specific biases related to true intake and cor related person-specific biases. Data were obtained from a dietary assessmen t validation study carried out among 160 women at the Dunn Clinical Nutriti on Center, Cambridge, United Kingdom, in 1988-1990. Using the biomarker mea surements and dietary report measurements from this study, the authors comp are the new model with alternative measurement error models proposed in the literature and demonstrate that it provides the best fit to the data. The new model suggests that, for these data, measurement error in the FFQ could lead to a 51% greater attenuation of true nutrient effect and the need for a 2.3 times larger study than would be estimated by the standard approach. The implications of the results for the ability of FFQ-based epidemiologic studies to detect important diet-disease associations are discussed.