Purpose: We discuss fundamental assumptions required for the validation of dietary questionnaire assessments, using latent variable models.
Methods: We discuss some methodological aspects of latent variable modelling in terms of the triad method, for comparisons between questionnaire assessments, food consumption records and/or biomarkers. In particular, we address the recent proposal (Frazer et al) that biases due to correlated random errors between different measurement types could be overcome by using two biomarkers as instrumental variables, as recently proposed by.
Results: The instrumental variable approach proposed by Frazer et al, but also other validation models proposed so far, require the assumption that questionnaire measurements are specific for a defined type of food or nutrient intake (latent variable of interest) - that is, conditionally on the true intake levels of this food or nutrient the questionnaire measurements should have no association with intakes of any other dietary component.
Conclusion: More methodological research is needed on the design of multivariate validation studies that might allow the examination of measurement specificity in practice.