Data Sources and Analytical Methods

This appendix presents selected details of the methods used to estimate the food and nutrient intakes of the children and adults.

Foods that are not consumed almost daily present special challenges for estimating usual intakes. If an item is consumed once a week or even less often, then the chances that a respondent will report a positive consumption during one or both of the two survey days is quite small. As a result, the observed daily intakes include a large proportion of zeroes, some of which are “real” (corresponding to the never-consumers) and some of which are “coincidental” (corresponding to the consumers who did not consume the item during the survey days).

Even if the committee did not attempt to separate the real from the coincidental zeroes, the spike in the observed intake distribution introduces some challenges that are difficult to address when attempting to estimate distributions of usual intakes. In particular, this type of intake data violates some of the assumptions that underlie the Iowa State University (ISU) method (Nusser et al., 1996) for estimating usual nutrient intake distributions.

Two approaches have been proposed to address this problem. In 1996, researchers at ISU (Nusser et al., 1996) proposed an extension of the ISU method for nutrients that can be implemented on items that are not consumed almost daily. Much later, in 2006, the National Cancer Institute

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.

Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 229

G
Data Sources and Analytical Methods
This appendix presents selected details of the methods used to estimate
the food and nutrient intakes of the children and adults.
USUAL FOOD INTAKE ESTIMATION
(ESTIMATION OF EPISODICALLY CONSUMED ITEMS)
Foods that are not consumed almost daily present special challenges for
estimating usual intakes. If an item is consumed once a week or even less
often, then the chances that a respondent will report a positive consumption
during one or both of the two survey days is quite small. As a result, the
observed daily intakes include a large proportion of zeroes, some of which
are “real” (corresponding to the never-consumers) and some of which are
“coincidental” (corresponding to the consumers who did not consume the
item during the survey days).
Even if the committee did not attempt to separate the real from the
coincidental zeroes, the spike in the observed intake distribution intro-
duces some challenges that are difficult to address when attempting to
estimate distributions of usual intakes. In particular, this type of intake data
violates some of the assumptions that underlie the Iowa State University
(ISU) method (Nusser et al., 1996) for estimating usual nutrient intake
distributions.
Two approaches have been proposed to address this problem. In 1996,
researchers at ISU (Nusser et al., 1996) proposed an extension of the ISU
method for nutrients that can be implemented on items that are not con-
sumed almost daily. Much later, in 2006, the National Cancer Institute
229

OCR for page 229

230 CHILD AND ADULT CARE FOOD PROGRAM
(NCI) proposed a different approach to estimate the usual intake distribu-
tion of episodically consumed items (Tooze et al., 2006). Both methods have
advantages and disadvantages.
The ISU method to estimate the distribution of episodically consumed
foods assumes (sometimes unrealistically) that the probability that a person
will consume the item is independent of the amount of the item consumed.
This is clearly inappropriate for items such as milk and alcohol, where
it is clearly seen that persons who tend to consume the items more often
also tend to consume more of it when they do consume it. On the other
hand, the NCI approach does not easily lend itself to implementation in
nationwide food consumption surveys such as the National Health and
Nutrition Examination Survey, because incorporating survey weights into
the analysis requires additional programming and is quite intensive from a
computational viewpoint.
Here, the committee implemented the ISU foods method and tested
whether the correlation between probability of consumption and amount
consumed was significantly different from zero. If not, then the committee
used the estimates that are obtained from the Personal Computer-Software
Intake Distribution Estimation (PC-SIDE) Version 1 when the item is de-
clared to be a “food.” If the correlation was significantly different from
zero, then the committee implemented the method suggested by Tooze et al.
(2006) and then incorporated the survey weights using a parametric boot-
strap approach. In most cases, the probability of consumption of the item
and the amount consumed did not appear to be correlated and, therefore,
the ISU foods method was deemed to be appropriate.
ESTIMATION OF THE PREVALENCE OF INADEQUACY IN
AGGREGATED DIETARY REFERENCE INTAKE GROUPS
To estimate prevalence of inadequate intakes in a group that includes
a mix of individuals of different ages who have different average require-
ments for the nutrient, the analysis must be performed using an approach
that accounts for the differences. This situation applies to the 2–4-year and
the 5–13-year age groups used for developing the recommended Meal Re-
quirements for the Child and Adult Care Food Program (CACFP). Each of
these two age groups spans two Dietary Reference Intake (DRI) age groups.
The committee describes below two alternative approaches that were used
to combine groups.
The first approach is appropriate when the number of persons from
each DRI group in the combined group is large enough for estimation of
usual intakes. The second approach, which produces prevalence estimates
that closely approximate those obtained in the first approach, is more use-

OCR for page 229

231
APPENDIX G
ful when the combined group includes just a few individuals from each of
various DRI groups.
Approach 1
Suppose that the committee wished to estimate the prevalence of inad-
equate intakes of a nutrient with an Estimated Average Requirement (EAR),
in a group, and that they had n sample persons, all of whom have provided
at least two 24-hour food intake records. Furthermore, suppose that n1 of
those persons belong to one DRI group for which the EAR of the nutrient
is a units, and that n2 persons belong to a second DRI group for which the
EAR of the nutrient equals b units.
The approach the committee implemented in this report to estimate
the prevalence of inadequate intakes among the n individuals was the
following:
1. The committee first estimated the prevalence of inadequate intakes
among the n1 persons by computing the proportion of persons in
that subgroup with usual intakes below a. Let p1 denote the esti-
mated prevalence of inadequacy in subgroup 1.
2. The committee then estimated the prevalence of inadequate intakes
among the n2 persons by computing the proportion of persons in
the second subgroup with usual intakes below b. Let p2 denote the
prevalence of inadequacy in subgroup 2.
3. The prevalence of inadequacy in the aggregated group was then
estimated as the weighted average of the two prevalences, where
the weights were given by the relative sample sizes. If p denoted the
overall prevalence, then
n1 3 p1 1 n2 3 p2
p5
n1 1 n2
The percentiles of the usual intake distribution of the nutrient in the
aggregated group are estimated as usual, using the daily intakes reported
by persons in both subgroups together.
Approach 2
When the numbers n1 and n2 of persons in each of the two DRI groups
was small, the committee could not obtain a reliable estimate of the two
prevalences, p1 and p2. In this case, they could still estimate the prevalence
of inadequate intakes in the aggregated group as follows:

OCR for page 229

232 CHILD AND ADULT CARE FOOD PROGRAM
1. First, select the larger of the two subgroups as the reference group.
For example, suppose that n1 . n2. If so, then subgroup 1, with
EAR equal to a, is the reference group.
2. Next, scale the daily intakes of persons in subgroup 2, by multi-
plying them by the factor a/b. By doing this, the committee made
the intakes from group 2 comparable to those from subgroup 1.
For example, if a 5 50 and b 5 55, then to compare intakes in
subgroup 2 to the EAR in the reference group, the committee de-
creased them by the factor 50/55.
3. Finally, estimate the proportion of usual intakes below the EAR in
the reference subgroup (in our example, subgroup 1) by carrying
out calculations using the observed daily intakes in subgroup 1 and
the rescaled daily intakes in subgroup 2.
This approximate approach works better when the difference in the
EARs in the subgroups is not large (so that the rescaling factor is not much
different from 1) and when there is a clearly larger subgroup in the combined
group. Both approaches extend in a straightforward manner to the case
where the aggregated group consists of more than two DRI subgroups.
PREVALENCE OF IRON INADEQUACY
The distribution of iron requirements in women of child-bearing age
is hypothesized to be skewed to the right. As a consequence, the EAR
cut-point method cannot be used to estimate the prevalence of inadequate
intakes of iron in adolescent and premenopausal women (IOM, 2000).
Instead, the probability of iron inadequacy is estimated using the full prob-
ability method (NRC, 1986).
For adolescent women ages 14–18 years and for adult premenopausal
women ages 19–30 years, the committee estimated the prevalence of iron
inadequacy by comparing the women’s usual iron intakes to established
at-risk thresholds (see IOM, 2001, Tables I-5 and I-6). In the case of young
women ages 11–13 years, the committee assumed that young women were
already menstruating and adjusted the risk thresholds corresponding to
14–18-year-olds by shifting the thresholds downward by 0.45 mg. In all
cases the committee assumed that the population of women included a mix
of oral contraceptive users and nonusers.
To increase the reliability of the prevalence estimate, the committee
proceeded as follows:
1. For each DRI subgroup, the committee estimated the parameters
of the usual iron intake distribution and generated 5,000 “typical”
usual intakes from each of those distributions.

OCR for page 229

233
APPENDIX G
2. Using the typical intakes, the committee estimated prevalence as
the average of the 5,000 risks, obtained empirically by comparing
the usual intake to the corresponding risk levels.
3. The subgroup specific prevalences were then averaged, using the
relative sizes of each DRI subgroup as the weight in the average.
REFERENCES
IOM (Institute of Medicine). 2000. Dietary Reference Intakes: Applications in Dietary Assess-
ment. Washington, DC: National Academy Press.
IOM. 2001. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium,
Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc.
Washington, DC: National Academy Press.
NRC (National Research Council). 1986. Nutrient Adequacy: Assessment Using Food Con-
sumption Surveys. Washington, DC: National Academy Press.
Nusser, S. M., A. L. Carriquiry, K. W. Dodd, and W. A. Fuller. 1996. A semiparametric
transformation approach to estimating usual daily intake distributions. Journal of the
American Statistical Association 91(436):1440–1449.
Tooze, J. A., D. Midthune, K. W. Dodd, L. S. Freedman, S. M. Krebs-Smith, A. F. Subar,
P. M. Guenther, R. J. Carroll, and V. Kipnis. 2006. A new statistical method for estimat-
ing the usual intake of episodically consumed foods with application to their distribution.
Journal of the American Dietetic Association 106(10):1575–1587.

OCR for page 229