Repeatability of food frequency assessment tools in relation to the number of items and response categories included
Background: Accuracy of a measurement is a cornerstone in research in order to make robust conclusions about the research hypothesis. Objective: To examine whether the number of items (questions) and the number of responses of consumption included in nutritional assessment tools influence their repeatability. Methods: During 2009, 400 participants (250 from Greece, 37±13 yrs, 34% males and 150 participants from Spain, 39±17 yrs, 41% males) completed a diet index with 11-items and binary (yes/no) responses, a diet-index with 11-items and 6-scale responses, a 36-item and a 76-item food frequency questionnaire (FFQ) with 6-scale responses. Participants completed these tools, twice, within 15-days period. Spearman-Brown (rsb), Kendall¿s tau coefficients and the Bland-Altman method were applied to answer the research hypothesis. Results: The highest repeatability coefficient was observed for the 11-items with binary responses index (rsb=0.948, p<0.001), followed by the 11-items with 6-scale responses index (rsb=0.943, p<0.001), the 36-item (rsb=0.936, p<0.001) and the 76-item FFQs (rsb=0.878, p<0.001). Statistical comparisons revealed no significant differences between repeatability coefficients of the first three tools (p>0.23); whereas the aforementioned tools had significantly higher repeatability coefficients as compared with the 76-item FFQ (p=0.002). Sub-group analyses by gender, education, smoking and clinical status, confirmed the aforementioned results. Conclusion: Repeatability has been revealed for all food frequency assessment tools used, irrespective of the number of items or the number of responses included.