Sampling variability and uncertainty in total diet studies

Here, the uncertainty budget for a total diet study (TDS) was clarified by separating the total measurement uncertainty into the uncertainty arising from the compositional heterogeneity of food items between cities (referred to as inter-city variance), the heterogeneity of food items within cities (intra-city variance), and the chemical analysis of the food samples (analytical variance) at one study design. TDS samples were collected from 14 cities in Japan. Duplicate samples collected in each city were prepared from food items purchased from different shops, and the cadmium concentrations were measured individually to obtain the intra-city variance. These results were used to show the importance of sampling design in TDSs, by evaluating a sampling method known as a multi-stage design, in which multiple samples are collected from several cities. Such schemes have been applied to TDSs, but the uncertainty involved has not been assessed. An intra-city correlation was observed between the cadmium concentrations in samples from the same city, demonstrating that the effective sample size was not simply the number of cities and shops sampled. The TDS results showed a high intra-city variance, which was greater than the inter-city variance for all of the food groups studied, and particularly for the bean and potato groups. By combining the sampling and analytical uncertainties obtained, the sampling uncertainty across different primary sampling unit sizes and secondary sampling unit sizes was obtained. As suggested by the analysis of potatoes and beans, grouping food samples from different shops in the same city can improve the representativeness of the results. This study examined the uncertainty budget, i.e. statement of a measurement uncertainty, of the components of that measurement uncertainty, and of their calculation and combination (VIM definition), for a nation-wide food survey, known as a total diet study. The total measurement uncertainty was separated into the uncertainty arising from the heterogeneity attributable to food from different cities (quantified as the inter-city variance), from heterogeneity within cities (intra-city variance), and from chemical analysis used (analytical variance).