Toward reliable estimation of servings of fruit and vegetables and fat practices from adults' 7-day food records

The objective of this study was to assess the assumption of compound symmetry and draw implications for reliably assessing among adults the number of servings of fruit and vegetables consumed and high-fat practices per day using daily food records. A cross-sectional sample of 153 female elementary school teachers (3rd to 5th grades) completed a 7 consecutive day food record. The main outcome measures included number of servings of fruit and vegetables, low-fat practices, and high-fat practices per day. Three models were tested for between-day correlations (compound symmetric, heterogenous compound symmetric, and unstructured);an intraclass correlation was calculated across days for reliability; the number of days of recording necessary to achieve a reliability of .7, .8, or .9 was estimated; and implications of separate covariance structures for weekdays versus weekend days were explored. The compound symmetry model characterized weekdays for fruit and vegetables and the number of high-fat practices when a square root transformation was employed. Five days are adequate for assessing servings of fruit and vegetables for weekdays at a reliability of .8 but 4 weekend days are necessary for the same reliability on weekends. Seven days of recording are necessary to assess high-Eat practices at a reliability of .8. To attain a reliability of .8 or higher, 7 days of recording are necessary for estimating high-fat practices; 9 consecutive days, beginning on a Saturday, are necessary for fruit and vegetables. Because of the large proportion of days with zero values, caution must be used to estimate number of days needed to reliably estimate number of low-fat practices per day.