adequacy of the individual's intake would reflect the uncertainty about that individual's requirement for the nutrient.
Hypothetically, if an individual's requirement could be observed, then the first term in the expression for SDD would be zero, and the uncertainty would reflect only the fact that the individual's usual intake for the nutrient cannot be observed.
The three situations above are extreme and typically do not occur. A more common situation is when there is some information about the individual's daily intake (allowing for an estimate of ӯ) and some idea of the distribution of requirements in the group. For example, the median requirement (EAR) and the coefficient of variation (CV) of requirements might be known, allowing the SDr to be derived.
Suppose that a level of confidence of at least 85 percent is desired before concluding that an individual's usual intake is adequate. To find out how large the ratio D/SDD would need to be to reach this conclusion, compare the D/SDD to the z-values listed in a standard z-table (e.g., a value of 0.85 in the table corresponds to a z-value of 1). Thus, if the ratio D/SDD is approximately equal to 1, it can be concluded with an 85 percent level of confidence that the individual's usual intake is larger than the requirement. Selected z-values, corresponding to different levels of assurance, are given in Table B-1. The criterion for using the ratio D/SDD and the qualitative conclusions from the quantitative analysis can be summarized as follows:
If D/SDD is greater than 1, then there is reasonable certainty that the individual 's usual intake is adequate. In other words, it is reasonably certain that the unobservable true difference between the individual's usual intake and requirement (y − ρ) is positive and thus the individual's usual intake exceeds requirement.
If D/SDD is less than −1, then it is reasonably certain that the individual's usual intake is inadequate. In other words, the true difference between the individual's usual intake and requirement (y − ρ) is negative and thus the individual's usual intake is less than the requirement.
If D/SDD is anywhere between −1 and 1, it cannot be determined with certainty whether the individual 's intake is adequate or inadequate.
The criterion above is derived by using principles from hypothesis testing and construction of confidence intervals under normality