## Task 1: Key Concepts about Understanding Usual Intake and Day-to-Day
Variation in Dietary Intakes

Occasionally,
dietary research focuses on an acute exposure—that is, intake at a given
point in time—such as when tracking the outbreak of a food borne illness.
Generally, however, for most surveillance, epidemiologic, and behavioral
research purposes, dietary analyses are concerned with measuring
*usual* intake—that is, long-term average daily intake. This is
because dietary recommendations are intended to be met over time and
diet-health hypotheses are based on dietary intakes over the long term.

As noted in previous courses, the main instrument for gathering dietary
intake data in the NHANES is the dietary recall. Dietary recalls are rich
in details regarding every item consumed (when, how, how much, with what),
and for this reason are considered the main instrument for estimating food
and nutrient intakes for the population. However, because they only cover
one or two 24-hour periods, they represent only a snapshot in time, rather
than usual intake, and some of these snapshots are not typical diet days for
the individual.

Due to this day-to-day variation, one (or even a few) 24-hour recall(s)
cannot be considered as an accurate assessment of an individual’s true usual
intake. Therefore, because long-run, or usual, dietary intake is most often
the measurement of interest, statistical adjustments are often necessary.
One exception to this is that **the ***mean* of the *population’s *
distribution of usual intake can be estimated from a sample of individuals’
24-hour recalls, without sophisticated statistical adjustment. For more
advanced analyses, such as estimating the percentiles of a distribution,
sophisticated adjustment techniques are needed. See the Advanced Dietary
Analyses course for more information.

The problem of estimating usual intake from 24-hour
recalls can be thought of as a measurement error issue. For more
information on measurement error, see “Task 2: Key Concepts about
Understanding Measurement Error.”