## Task 2: Key Concepts about Estimating Distributions of Usual Intake for a Single Ubiquitously-consumed Dietary Constituent with a Few Days of 24-hour Recalls for Subpopulations using a Covariate

Because the dietary assessment queries intake only on a single day or a few days, measures of usual intake from 24-hour recalls are prone to measurement error. Using a simple average of 2 days does not adequately represent usual intake. Thus, more sophisticated methods based on statistical modeling are necessary. All of the statistical methods that have been developed make the assumption that the 24-hour recall is prone to random, not systematic error. For estimating ubiquitously-consumed dietary constituents, they must meet the following challenges. The methods must:

- Distinguish within-person from between-person variation, and
- Account for consumption-day amounts that are positively skewed.

The methods developed before the NCI method required stratification to estimate the distribution of usual intake for a subpopulation (see Module 18, Task 2 for more details on these methods). Because it is able to accommodate covariates in the statistical model, the NCI method can provide more efficient estimates (i.e., smaller standard errors) of the distribution of usual intake for a subpopulation than these other methods compared to fully stratified models that do not share parameters. This Task addresses the estimation of subpopulations defined by a covariate. This method could be used when estimating usual intake for multiple groups is of interest. When just one subpopulation is of interest, the methods described in Task 1 could be used. Balanced Repeated Replication (Module 18, Task 2) is used to calculate standard errors that are corrected for the complex sampling design of NHANES.

The macros to fit the NCI method may be downloaded from the NCI website.