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Dietary Reference Intakes: Applications in Dietary Assessment (2000) Institute of Medicine (IOM)

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. "Appendix C: Assessing Prevalence of Inadequate Intakes for Groups: Statistical Foundations." Dietary Reference Intakes: Applications in Dietary Assessment. Washington, DC: The National Academies Press, 2000.

 Page 208

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DRI DIETARY REFERENCE INTAKES: Applications in Dietary Assessment

ued to assess the extent to which an incorrect model specification may affect the properties of prevalence estimates.

THE EAR CUT-POINT METHOD

The probability approach described in the previous section is simple to apply and provides unbiased and consistent estimates of the prevalence of nutrient inadequacy under relatively mild conditions (i.e., intake and requirement are independent, distribution of requirement is known). In fact, if intakes and requirements are independent and if the distributions of intakes and requirements are known, the probability approach results in optimal (in the sense of mean squared error) estimates of the prevalence of nutrient inadequacy in a group. However, application of the probability approach requires the user to choose a probability model (a probability distribution) for requirements in the group. Estimating a density is a challenging problem in the best of cases; when data are scare, it may be difficult to decide, for example, whether a normal model or a t model may be a more appropriate representation of the distribution of requirements in the group. The difference between these two probability models lies in the tails of the distribution; both models may be centered at the same median and both reflect symmetry around the median, but in the case of t with few degrees of freedom, the tails are heavier, and thus one would expect to see more extreme values under the t model than under the normal model. Would using the normal model to construct the risk curve affect the prevalence of inadequacy when requirements are really distributed as t random variables? This is a difficult question to answer. When it is not clear whether a certain probability model best represents the requirements in the population, a good alternative might be to use a method that is less parametric, that is, that requires milder assumptions on the t model itself. The Estimated Average Requirement (EAR) cut-point method, a less parametric version of the probability approach, may sometimes provide a simple, effective way to estimate the prevalence of nutrient inadequacy 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 demanding symmetry requirement and therefore choosing between one or the other becomes an unnecessary step.

The cut-point method is very simple: estimate prevalence of inad-

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 Front Matter (R1-R14) Contents (R15-R18) Summary (1-18) I. Historical Perspective and Background (19-20) 1 Introduction and Background (21-28) 2 Current Uses of Dietary Reference Standards (29-42) II. Application of DRIs for Individual Diet Assessment (43-44) 3 Using Dietary Reference Intakes for Nutrient Assessment of Individuals (45-70) III. Application of DRIs for Group Diet Assessment (71-72) 4 Using the Estimated Average Requirement for Nutrient Assessment of Groups (73-105) 5 Using the Adequate Intake for Nutrient Assessment of Groups (106-112) 6 Using the Tolerable Upper Intake Level for Nutrient Assessment of Groups (113-126) 7 Specific Applications: Assessing Nutrient Intakes of Groups Using the Dietary Reference Intakes (127-144) IV. Fine-Tuning Dietary Assessment Using the DRIs (145-146) 8 Minimizing Potential Errors in Assessing Group and Individual Intakes (147-161) 9 Research Recommended to Improve the Uses of Dietary Reference Intakes (162-167) 10 References (168-178) Appendix A: Origin and Framework of the Development of Dietary Reference Intakes (179-184) Appendix B: Nutrient Assessment of Individuals: Statistical Foundations (185-202) Appendix C: Assessing Prevalence of Inadequate Intakes for Groups: Statistical Foundations (203-210) Appendix D: Assessing the Performance of the EAR Cut-Point Method for Estimating Prevalence (211-231) Appendix E: Units of Observation: Assessing Nutrient Adequacy Using Household and Population Data (232-238) Appendix F: Rationale for Setting Adequate Intakes (239-253) Appendix G: Glossary and Abbreviations (254-261) Appendix H: Biographical Sketches of Subcommittee Members (262-266) Index (267-281) Summary Table: Estimated Average Requirements (282-283) Summary Table: Tolerable Upper Intake Levels (284-286) Summary Table: Recommended Intakes for Individuals (287-289)