Dietary Reference Intakes Applications in Dietary Planning (2003) / Chapter Skim
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Appendix E: Adjustment of Observed Intake Data to Estimate the Distribution of Usual Intakes in a Group
Appendix E: Adjustment of Observed Intake Data to Estimate the Distribution of Usual Intakes in a Group
Pages 196-208
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From page 196... ...
This poses a major obstacle to the assessment of an incliviclual's nutrient intake because his or her usual intake is generally poorly estimated from only a few clays of observation, yet more extensive ciata collection is rarely feasible. Assessments of nutrient acloquacy among groups are facilitated by the availability of statistical adjustment procedures to estimate the clistribution of usual intakes from observed intakes, as long as more than one clay of intake ciata has been collected for at least a representative subsample of the group.
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From page 197... ...
THE NATIONAL RESEARCH COUNCIL METHOD Conceptual Underpinnings In assessing nutrient acloquacy it is necessary to estimate usual intake. However, usual intake cannot be inferred from measures of observed intake without error.
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From page 198... ...
This adjustment procedure clepencis on the properties of a normal distribution, yet the observed distribution of intakes for most nutrients is likely to be positively skewoci. This is because the clistribution is naturally truncated at 0 (i.e., reported intakes cannot fall below this value)
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From page 199... ...
/n where n is the mean number of clays of intake ciata per subject in the sample. VbeGween represents the "true" variance of the distribution of usual intakes.
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From page 200... ...
If the ciata were not transformed, however, the acljusteci intakes calculated from this equation now represent the estimated distribution of usual intakes.
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From page 201... ...
Normality As noted earlier, the NRC method hinges on having normally clistributeci intake ciata or being able to transform the observed ciata into a normal distribution. If nonnormal data are not transformed prior to adjustment, or if the applied transformation fails to correct for the nonnormality of the ciata, then assessments of the preva
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From page 202... ...
. In cases where the ciata fail to satisfy the assumptions of a normal distribution even when transformed, application of the NRC method and use of the resultant acljusteci distribution for nutrient assessment is problematic (Beaton et al., 1997~.
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From page 203... ...
The distribution of usual intakes is then estimated from this distribution of transformed intake values and the estimates are mapped back to the original scale through a bias-acljusteci back transformation. The procedures represent a major advance over the NRC method and a number of other more complicated adjustment procedures that have been proposed (Hoffmann et al., 2002~.
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From page 204... ...
Despite its strengths, however, the ISU method may not be as appropriate as the NRC method for use with small samples. The greater complexity of the ISU method requires a larger sample to ensure that the various steps in the adjustment procedure retain acceptable levels of reliability.
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From page 205... ...
Underlying Assumptions and [imitations of Adjustment Methods One important difference in application of the two methods clescribeci here is that the ISU method of adjustment is typically applied to the distribution of intakes on clay one of ciata collection, whereas the NRC method is applied to multiple-clay means. In the design of large clietary surveys it is becoming increasingly common to collect a second clay of intake ciata on only a subsample of the group.
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From page 206... ...
Systematic errors in the estimation of nutrient levels in foocis consumed will bias the estimated clistribution of usual intakes. In the case of nutrients for which food composition ciata are known to be incomplete, analysts must gauge the extent to which reported intakes will be biased.
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From page 207... ...
. If the original data are transformed to better approximate a normal distribution, this program should be run on the transformed data and the final adjusted data backtransformed prior to the assessment of nutrient adequacy or excess.
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