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5. The Probability Approach
Pages 25-40

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From page 25...
... This method takes into account the likelihood that persons with a particular level of intake would fail to meet their nutrient requirement. The probability of inadequate intake would naturally be very low for those with higher nutrient intakes and would be higher for those with lower nutrient intake.
From page 26...
... In Figure 5-1, the probability of inadequate intake is plotted against the protein intake for an adult male. Beyond approximately 30 g/day, the probability of inadequate intake decreased rapidly and reaches zero at an intake of about 60 g/day.
From page 27...
... A specific example of the probability approach applied to protein intake is presented in Table 5-1. The adjusted distribution of protein intakes using 1977-1978 NFCS data provided to the subcommittee (Figure 4-2)
From page 28...
... fPercentages do not add to 100 due to rounding. gPrevalence of inadequate protein intake in this population of adult males.
From page 29...
... Figure S-2 portrays the probability, or risk, curve for inadequate iron intake by menstruating women, derived as described in Appendix B REQUIREMENT INFORMATION NEEDED FOR THE PROBABILITY APPROACH A knowledge of, or a reasonable assumption about, the mean and shape of the requirement distribution for a particular nutrient is necessary for the probability approach to be applied.
From page 30...
... This is done in the next section by testing the effect of changing the mean and the parameters of the requirement distribution on the estimate of prevalence. EFFECT OF REQUIREMENT DISTRIBUTION ON ESTIMATES OF THE PREVALENCE OF INTAKE ADEQUACY Influence of Mean and Standard Deviation of Requirement NFCS data on vitamin C intake of adult men can be used to illustrate how changes in the mean and the variability of the requirement distribution can affect estimates of the prevalence of inadequate intake.
From page 31...
... When mean requirement is held constant and the standard deviation is increased (across rows in Table 5-2) , the predicted prevalence estimates do not change substantially.
From page 32...
... Generally, therefore, the errors of overestimation and underestimation of the prevalence of inadequate intake tend to cancel out, and except at the ends of the intake distribution, the model is not sensitive to changes in the shape of the requirement distribution, i.e., its symmetry. These empirical findings seem to apply to all the intake distributions examined by the subc~'littee (ascorbic acid, protein, and vitamin A for adult males and females and iron, thiam~n, and thiamin/l,OOO local for adult males)
From page 33...
... Thus, the use of any reasonable standard deviation of requirement will improve the estimate of prevalence. The subcommittee also explored the use of the probability approach when the requirement distribution is highly asymmetrical, e.g., for iron requirements of menstruating women.
From page 34...
... Comparison of Probability Estimates of the Prevalence of Inadequate Iron Intake and the Proportion of Intakes Falling Below the Mean Requirementa Estimates Derived Using Median Require Inferred Estimates Assumed Median Derived f ram Limit of Requirement Probability ment a" Cutoff Absorption (96)
From page 35...
... For this reason, the simplified empirical approach in which the proportion below the median is used as an estimate of the prevalence of inadequacy cannot be applied to iron intake in menstruating women -- or to any other nutrient where there is reason to believe that there is strong asymmetry in the distribution. Impact of the Mode in Which Requirements Are Expressed It is generally accepted that nutrient requirement estimates, and hence approaches to estimating the prevalence of inadequate intakes, must take into account the physiological variables of age, sex, pregnancy, and lactation.
From page 36...
... Table 5-4 demonstrates that there is a substantial difference between the estimates derived in these two ways because in the second approach correlation between intake and requirement is avoided and there is recognition that a person with a low thiamin intake may also have a low energy intake and, hence, a low but adequate requirement for thicken. In the ~ ~ - ~ 1980 Recommended Dietary Allowances, the proposed thiamin allowance was expressed as 0.5 mg/1,000 kcal/day (NRC, 1980)
From page 37...
... As illustrated by the analysis of thiamin, when a known variable of requirement can be measured and applied in analysis, it is correct to express both intake and requirement in relation to this variable before applying the probability approach to assessment, If fully valid prevalence estimates are to be obtained. Impact of Criteria for Requirement Estimate As discussed earlier, the criteria that serve as the conceptual framework for the requirement estimate have a substantial effect on estimates of the prevalence of inadequate intake.
From page 38...
... To illustrate the importance of changes in nutrient requirement, an TABLE 5-5. Prevalence of Inadequate Intake Estimated with Two Different Assumptions About Requirement Distributions of Adult Mena Criterion of Adequacy for Requirement Estimated Prevalence of Inadequate Intakes (I)
From page 39...
... TABLE 5-6. Comparison of Estimates of the Prevalence of Inadequate Intakes for Adults Using Probabll ity and Fixed Cutoff Approachesa Prevalence Estimates ( 9e )
From page 40...
... SU=A" When the shape of the distribution of nutrient reguirement is known or can be inferred, a probability approach to the assessment of observed nutrient intakes is the most efficient and logical analytical approach. If, as is a reasonable assumption for most nutrients, the requirements are distributed relatively symmetrically about the mean or median, the probability approach is sensitive to the estimate of the average requirement.


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