Objective: The study was conducted to assess the reproducibility and validity of a 190-item self-administered quantitative food frequency questionnaire, used in a nation-wide study of adolescents.
Design and subjects: Reproducibility study; 103 11th grade students (18 years) completed the questionnaire twice, with a 6-week interval. Validation study; 49 11th grade students filled in the questionnaire and kept 7-day weighed food records.
Results: Spearman rank correlations between the nutrient intakes from the two questionnaires varied from 0.63 (sugar energy percentage) to 0.91 (alcohol). The median coefficient was 0.85. The first questionnaire produced generally higher nutrient intake estimates than the second. Correlations between nutrient intake values from the records and the questionnaire ranged from 0.14 (vitamin D, non-significant, cod liver oil not included) to 0.66 (monounsaturated fatty acids). The median coefficient was 0.52. Adjustment for energy intake did not materially affect the correlations. On average 41% of the subjects were classified in the same quartile in the questionnaire and the records, and 2% in the opposite quartiles. However, the percentage of subjects in the same/opposite quartiles for vitamin D and fibre were 33/12 and 22/6, respectively.
Conclusions: It is concluded that the questionnaire is able to rank subjects according to the tested nutrients, except vitamin D and fibre.