| Food Composition Data: A User's Perspective (1987) |
|Consideration of food composition variability: What is the variance of the estimate of one-day intakes? Implications for setting priorities|
In another setting the author has been concerned with approaches to the nutritional assessment and interpretation of population data . Two problems have been identified in this work: day-today variation in intake must be taken into account or there can be potentially serious errors in the estimation of the prevalence of either inadequate or excessive intakes; and the nutritional adequacy of intake (or the risk of excess associated with detrimental factors) must be approached on a probability basis. With replicated observations the first of these problems may be addressed by statistical adjustment of the distribution following ANOVA to estimate the partitioning of variance. The second can be addressed by generating probability statements based upon the distribution of requirements among individuals [ 1, 10, 6].
In the course of developing specific approaches applicable to large-scale surveys, consideration was directed to the question of food composition "errors." Current USDA tables provide estimates of the standard error associated with the average content figures for individual foods . Standard deviations can be derived. These appear very large, with coefficients of variation (CV) ranging from about 10 to 50 per cent depending upon the food and nutrient. At first it appeared that this error, whether real (i.e. methodologic) or simply due to the range of compositions that a particular sample of a class of food might have (biological variation), was so large that any approach to assessment might be in jeopardy. To examine this specific issue, some examinations of the predicted "error" of one-day intakes were undertaken. The results of these examinations and considerations of implications are presented below.
Table 1. Variability of food composition as empirically estimated from USDA composition tablesa
CV range assumed
|Below cut-off||Above cut-off|
|Calcium||20 mg/ 100 g||5-50||5-15|
|Iron||1 mg/ 100 g||5-65||10-30|
|Riboflavin||0.05 mg/100 g||5-50||10-30|
|Vitamin B6||0.1 mg/100g||5-50||10-30|
|Vitamin A||30 IU/100g||5-65||10-30|
a. CVs estimated from reported SE of the mean value presented in table and reported number of determinations for the food .