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daily intakes for an individual are typically not available to reliably estimate the day-to-day variability, the pooled day-to-day SD of intake from CSFII (see Table B-2, Table B-3, Table B-4 through Table B-5) or from NHANES is used.

The z-statistic is constructed as follows:

z = × (observed mean intake − AI)/SD of daily intake.

By rearrangement, this can also be expressed as:

z = (observed mean intake − AI) / (SD of daily intake/ ).

The z-statistic is then compared to tabulated values (a selection of which are presented in Table B-6), to decide whether the desired level of accuracy is achieved when stating that the usual intake is larger than the AI.

For example, consider a nutrient such as calcium with an AI of 1,000 mg /day, and suppose that the SD of daily intake from CSFII for the appropriate life stage and gender group is 325 mg/day.

TABLE B-6 Selected Values of z and the Associated Level of Confidence When Concluding That Individual Usual Intake Is Larger Than the Adequate Intake (AI) or Less Than the Tolerable Upper Intake Level (UL)

Criterion

Conclusion

Probability of Correct Conclusion

z > 2.00

Usual intake is adequate (excessive)

0.98

z > 1.65

Usual intake is adequate (excessive)

0.95

z > 1.50

Usual intake is adequate (excessive)

0.93

z > 1.25

Usual intake is adequate (excessive)

0.90

z > 1.00

Usual intake is adequate (excessive)

0.85

z > 0.85

Usual intake is adequate (excessive)

0.80

z > 0.68

Usual intake is adequate (excessive)

0.75

z > 0.50

Usual intake is adequate (excessive)

0.70

z > 0.00

Usual intake is adequate (excessive/safe)

0.50

z > −0.50

Usual intake is adequate (excessive)

0.30 (0.70 probability usual intake is safe)

z > −0.85

Usual intake is adequate (excessive)

0.20 (0.80 probability usual intake is safe)

z > −1.00

Usual intake is adequate (excessive)

0.15 (0.85 probability usual intake is safe)

SOURCE: Adapted from Snedecor and Cochran (1980).



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