Correction and Revision of Conclusions — Dietary Trends in the United States

Abstract

The article by Popkin et al. (Sept. 5, 1996, issue)¹ on dietary trends presents a potentially misleading picture of fruit and vegetable intake in the United States. The implied serving sizes (grams of dietary intake in Table 4 of the article divided by numbers of servings in Table 3) range from 33.7 to 56.9 g (1.2 to 2.0 oz), lower than weights corresponding to recommended serving sizes (2 to 6 oz). In addition, the reported numbers of servings in Table 3 (4.3 to 4.8 for whites and 3.9 to 5.4 for blacks) generally exceed and run counter to those previously reported for the same survey (4.4 for whites and 3.9 for blacks).² These differences are not easily explained by the information provided. For national estimates to be valid, sample weights should be used to adjust for nonresponse and for the different rates of selection.³ It appears that such weights were used in only one table. Furthermore, it appears that the authors did not use specialized software that adjusts for the clustered nature of the sample in the calculation of standard errors.³ Therefore, the standard errors reported may be too small and result in tests of significance indicating real differences when, in fact, the differences might be due to sampling variation. Finally, the use of a single 24-hour dietary recall to represent the distribution of intake is problematic. Because persons with extreme intakes on the recall day would receive higher or lower scores than their usual intake would merit, the proportions of both persons meeting the recommendations and those with very poor diets are likely to be overestimated in Table 2.