Objective: To determine the accuracy of direct observation in food-weight estimation as measured under controlled field conditions.
Design and subjects: Ten local Nepalis were trained in observational techniques and tested in food-weight estimation during a 3-month training period and for 4 months after training.
Setting: The study was carried out in the Sarlahi District, a rural, central lowland region of Nepal that borders India.
Main outcome measures: Thirty testing sessions (a total of 6,902 observations) were completed on more than 150 different foods. Estimates of observed food weight were compared with actual weights and were analyzed.
Statistical analyses performed: Pearson's correlation coefficients were calculated to examine associations between estimated and actual weights.
Results: Observer estimates of food weights were highly correlated with actual weights (r = .96) for the entire testing period. The linear regression equation (y = .96x + 1.3) suggests that the relationship between actual and observed food weights (in grams) was also accurate. Most observers showed improvement with training. Substantial reductions in both mean and standard deviation of percentage error were achieved over time. Accuracy of estimates was influenced by characteristics of foods weighted; small quantities (less than 20 g), certain nonstaple foods, and foods of high volume but light weight had less accurate estimates.
Conclusions: Direct observation is an important method for assessing dietary intake that does not rely on a respondents' ability to recall his or her own or another's food consumption. It is feasible to train local observers to make visual estimates of food weight, but the accuracy of their estimates varies by food and portion size.