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Appendix C: Assessing Prevalence of Inadequate Intakes for Groups: Statistical Foundations
Pages 203-210

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From page 203...
... In the remainder of this appendix, usual intakes and usual requirements are simply referred to as intakes and requirements, respectively. The problem of interest is assessing the proportion of individuals in the group with inadequate intake of the dietary component.
From page 204...
... The problem is not the actual probability calculation but rather the estimation of the joint distribution of intakes and requirements in the population. To recluce the ciata burden for estimatingfyR, approaches such as the probability approach proposed by the National Research Council (NRC, 1986)
From page 205...
... If that estimated usual intake distribution places a very high probability on intake values less than 90, then one would con1 When the distribution of requirements is approximately normal, the cdf can be easily evaluated in the usual way for any intake level a. Let z represent the standardized intake, computed as z= (a- mean requirement/D, where SD denotes the standard deviation of requirement.
From page 206...
... In this case, estimating the portion of inclivicluals likely to have inacloquate intakes is equivalent to computing a weighted average of risk, as explained below. The quantity of interest is not the risk associated with a certain intake level but rather the expected risk of inadequacy in the population.
From page 207...
... The NRC report discusses the effect of incorrectly specifying the form of the requirement distribution on the performance of the probability approach to assess prevalence (see pages 32-33 of the 1986 NRC report) , but more research is needed in this area, particularly on nonsymmetrical distributions.
From page 208...
... cut-point method, a less parametric version of the probability approach, may sometimes provide a simple, effective way to estimate the prevalence of nutrient inacloquacy in the group even when the underlying probability model is difficult to determine precisely. The only feature of the shape of the underlying model that is required for good performance of the cut-point method is symmetry; in the example above, both the normal and the t models would satisfy the less clemancling symmetry requirement and therefore choosing between one or the other becomes an unnecessary step.
From page 209...
... Because inferences are based on joint rather than the univariate distributions, an incliviclual consuming a nutrient at a level below the mean of the population requirement may be satisfying the incliviclual's own requirements. That is the case for all the inclivicluals represented in Figure 4-S by points that appear below the 45° line and to the left of the vertical EAR reference line, in triangular area B
From page 210...
... = Fy (EAR) The prevalence of inacloquate intakes can be assessed as long as one has an estimate of the usual nutrient intake distribution (which is almost always available)


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