Background: As part of the European Prospective investigation into Cancer and Nutrition (EPIC), preliminary studies were conducted to evaluate the accuracy of individuals' dietary intake measurements from newly developed questionnaires.
Methods: In six countries that adhered from the very beginning to the multicentre, co-ordinated EPIC project, the validity studies were based on two repeat questionnaire measurements at the start and at the end of a 1-year period, in groups of about 100 volunteers of both sexes. In addition, during this year, up to 12 24-hour recalls per person were taken monthly, and up to four blood and urine specimens were collected for measurement of biochemical markers. In three countries that joined EPIC later, the designs of the validity studies and type of 'reference' measurement chosen were somewhat different. The results presented in this overview paper are taken partly from more detailed, country-specific publications, and partly from a central (re-)analysis of the original data, to ensure a uniform approach to the statistical analyses and presentation.
Results: Averaged over subgroups by country and gender, Spearman coefficients of correlation between questionnaire measurements and the individuals' average 24-hour recalls ranged from 0.37 for fish to 0.68 for dairy products and 0.79 for alcoholic beverages. For energy-adjusted nutrient intakes (or nutrient densities, in the UK), mean Pearson correlation coefficients, corrected for residual attenuation due to day-to-day variations in the 24-hour recalls in all but two countries, ranged from 0.37 for retinol and 0.48 for vitamin E to 0.60 for carbohydrates and 0.12 for total alcohol intake. Correlations between energy-adjusted nutrient intakes and biochemical markers on average were low, but varied considerably between study centres.
Conclusions: On average, most estimated correlation coefficient were of similar magnitude to those observed by independent research groups. The role of the preliminary validity studies, and various benefits drawn from these studies for further planning of the EPIC project are discussed.