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TABLE 3-2 Illustration of the Computations Necessary to Test Whether Usual Intake Is Above the Adequate Intake (AI) for Different Numbers of Days of Observed Intake for a Woman 40 Years of Age

 

Using SD from CSFIIa

If SD is 25 Percent Larger

If SD is 50 Percent Larger

Mean intake

1,200 mg

1,200 mg

1,200 mg

SD of intakeb

325 mg

406 mg

488 mg

AI for calciumc

1,000 mg

1,000 mg

1,000 mg

z-Values = (mean intake – AI)/(SD/square root [n])

1 d of intake

0.61

0.49

0.41

3 d of intake

1.07

0.85

0.71

7 d of intake

1.69

1.30

1.08

Percentage confidence that the woman's usual intake exceeds the AId

1 d of intake

73

69

66

3 d of intake

86

80

76

7 d of intake

95

90

86

NOTE: The confidence with which one can conclude that usual intake is greater than the AI decreases when the number of days of daily intake records for the individual decreases, or when the SD of daily intake increases.

aSD = standard deviation; CSFII = Continuing Survey of Food Intake by Individuals,

bSD of calcium intake for women 19 through 50 years of age taken from CSFII (Appendix Table B-2).

c Adequate Intake for women 31 through 50 years of age.

d Confidence values were taken from a standard z-table (Snedecor and Cochran, 1980). The z-table is used because the SD of daily intake is assumed to be known (e.g., from CSFII), and is not computed from the woman's daily observations.

intake records and different SDs of daily intake for calcium were assumed. For each case, the confidence with which one would conclude that her usual intake is above the AI was calculated and is shown in the table.

If one can conclude that in fact usual intake appears to be larger than the AI with desired accuracy, then there is considerable assurance that the individual's intake is adequate. However, if the test does not result in the conclusion that usual intake is larger than the AI with the desired precision, then it cannot be inferred that intake is inadequate.

As discussed earlier, this approach is not appropriate when daily intakes for an individual are not approximately normally distributed.



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