Objective: To study the relative validity of an open-ended question on the consumption of dietary supplements in the preceding five-year period, incorporated in a self-administered questionnaire used in the NLCS, The Netherlands Cohort Study on diet and cancer (120,852 men and women aged 55-69 years).
Design: Questionnaire data were compared with reference information from three personal interviews carried out within a period of 10 months.
Setting and subjects: A randomly selected subgroup (59 men and 50 women) of the cohort living in 12 municipalities in the eastern and western regions of The Netherlands.
Results and conclusions: The overall sensitivity of the questionnaire concerning the use of any dietary supplement was 65.9%, the specificity was 98.5%; kappa as measure of agreement was estimated at 0.69. A high percentage recall was observed among women, users of at least three types of dietary supplements, long-term supplement users and those in the oldest age group. Recall of intake of specific supplements ranged from 77.8% for garlic preparations to 11.8% for 'other' supplements. Estimates of consumption of specific supplements (garlic and vitamin preparations) may provide enough precision to correctly classify individuals as users or non-users of those supplements.