title: Sample size and repeated measures required in studies of foods in the homes of African-American families.
abstract: Measurement of the home food environment is of interest to researchers because it affects food intake and is a feasible target for nutrition interventions. The objective of this study was to provide estimates to aid the calculation of sample size and number of repeated measures needed in studies of nutrients and foods in the home. We inventoried all foods in the homes of 80 African-American first-time mothers and determined 6 nutrient-related attributes. Sixty-three households were measured 3 times, 11 were measured twice, and 6 were measured once, producing 217 inventories collected at ~2-mo intervals. Following log transformations, number of foods, total energy, dietary fiber, and fat required only one measurement per household to achieve a correlation of 0.8 between the observed and true values. For percent energy from fat and energy density, 3 and 2 repeated measurements, respectively, were needed to achieve a correlation of 0.8. A sample size of 252 was needed to detect a difference of 25% of an SD in total energy with one measurement compared with 213 with 3 repeated measurements. Macronutrient characteristics of household foods appeared relatively stable over a 6-mo period and only 1 or 2 repeated measures of households may be sufficient for an efficient study design.
authors.affiliation: Department of Nutrition, Gillings School of Global Public Health, University of North Carolina, Chapel Hill, NC, USA. June_Stevens@unc.edu
authors.forename: June
authors.initials: J
authors.lastname: Stevens
authors.forename: Maria
authors.initials: M
authors.lastname: Bryant
authors.forename: Chin-Hua
authors.initials: CH
authors.lastname: Wang
authors.forename: Jianwen
authors.initials: J
authors.lastname: Cai
authors.forename: Margaret E
authors.initials: ME
authors.lastname: Bentley
mesh_terms: D000328:Adult; D001741:Black or African American; D002149:Energy Intake; D005190:Family / Q000208:ethnology; D005260:Female; D005502:Food / Q000145:classification*; D005504:Food Analysis; D005523:Food Supply / Q000706:statistics & numerical data*; D006801:Humans; D009749:Nutrition Surveys / Q000379:methods*; D012107:Research Design; D018401:Sample Size