Assessing usual dietary intake in complex sample design surveys: the National Dietary Survey

Abstract

The National Cancer Institute (NCI) method allows the distributions of usual intake of nutrients and foods to be estimated. This method can be used in complex surveys. However, the user must perform additional calculations, such as balanced repeated replication (BRR), in order to obtain standard errors and confi dence intervals for the percentiles and mean from the distribution of usual intake. The objective is to highlight adaptations of the NCI method using data from the National Dietary Survey. The application of the NCI method was exemplifi ed analyzing the total energy (kcal) and fruit (g) intake, comparing estimations of mean and standard deviation that were based on the complex design of the Brazilian survey with those assuming simple random sample. Although means point estimates were similar, estimates of standard error using the complex design increased by up to 60% compared to simple random sample. Thus, for valid estimates of food and energy intake for the population, all of the sampling characteristics of the surveys should be taken into account because when these characteristics are neglected, statistical analysis may produce underestimated standard errors that would compromise the results and the conclusions of the survey. 172SUsual dietary intake in complex sample Barbosa FS et al For most epidemiological diet analyses, usual intake is required, and in many situations, such as in large surveys, only one or two 24-hour recalls (24HR) or food records are collected. These methods yield an excessive amount of within-person variation, 4 and early attempts to compensate for this limitation by averaging over a small number of days 1 do not adequately represent usual individual intakes. Thus, more sophisticated methods based on statistical modelling were developed, 2 paying special attention to the problems that are inherent in modelling usual intake of foods or food groups that are episodically consumed. Challenges for the statistical modelling of usual intake include the following: the ratio of within-person and between-person variation; the reported days that are without consumption or the consumption-day amounts that are positively skewed, with extreme values in the upper tail; the correlation between the probability of consumption and the consumption-day amount; and covariate information on usual intake.The National Cancer Institute (NCI) method was designed to meet all of these challenges by allowing effi cient estimation of the usual intake distributions of daily and episodically consumed items. 13 The method also allows the prediction of individual intakes to be used in a model to assess the relationship between diet and disease or another variable 6 and by performing an assessment of the effects of individual covariates on consumption.12 An extension of the NCI method has also been used to estimate the population distributions of the ratios of usual intakes of dietary components. 5 RESUMOO objetivo do estudo foi indicar uma adaptação do método do Instituto Nacional do Câncer (INC), ut...

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