Some statistical considerations on dietary assessment methods.

Dietary intake for 18 nutrients in a sample of 230 free-living elderly people in Naples was assessed by three dietary assessment methods (diet history, 24-h recall and 3-d record). Different statistical criteria (the correlation coefficient, the mean difference and the proportion of individuals placed in the same thirds of distribution) were used to evaluate the agreement among different methods. Theoretical considerations are shown demonstrating the uselessness of the correlation coefficient in evaluating the agreement. In analysing the mean differences, diet history gave higher estimates of nutrient intake and 24-h recall gave lower estimates, when compared to the 3-d record. Large standard errors for the mean differences and small proportions of individuals placed in the same thirds of the distribution, for nearly all the nutrients examined, showed poor agreement among the three different methods. These findings strongly suggest the presence of bias in, at least, two of the three dietary assessment methods. Some statistical techniques for improving the diet record method in epidemiological studies are discussed: the need to transform nutrient intake data to achieve normality in the distribution; the number of days of recording required to produce a valid study; and the need to evaluate the bias in the diet record.