To date, epidemiologic studies have used more intensive self-report methods as the reference instrument for the calibration subsample, for example, multiple-day food records, as in the Health Professionals Follow-Up Study (2), or several recalls of food intake in the previous 24 hours. Cumulative evidence from studies using “recovery” biomarkers, such as doubly-labeled water, as a reference standard for total energy intake (recovery biomarkers do meet requirements for a reference instrument (10)) shows that these intensive self-report reference methods can have systematic errors that are correlated with food-frequency questionnaire errors, therefore violating a key requirement of reference instruments (11). Unfortunately, reliable recovery biomarkers are available for only a few dietary factors—in addition to doubly labeled water. Urinary nitrogen is a marker for protein intake, and urinary potassium may be a marker for dietary potassium intake (10). Therefore, in most studies, the use of self-report instruments in a calibration subsample as the reference standard is currently our best, although imperfect, approach to adjustment for measurement error.