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Agreement between interview data and a self-administered questionnaire on dietary supplement use.

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.