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of nutrient intakes also may be inaccurate if bioavailability varies within the population but is not considered when nutrient intake is estimated for each individual. Zinc, niacin, iron, and provitamin A carotenoids are nutrients with well-known issues of bioavailability. Nutrient equivalents are sometimes used (e.g., niacin equivalents for assessing niacin intake and retinol equivalents for assessing intakes of provitamin A carotenoids) (IOM, 1998b, 2000). The use of dietary folate equivalents to reflect the bioavailability of supplemental folate in contrast to folate naturally present in food has been recommended for evaluating dietary data (IOM, 1998b).

ISSUES OF VARIANCE IN DIETARY ASSESSMENT
Selecting a Representative Subsample of a Group

For large groups of people, it is not usually practical to assess the intake of every individual. Thus, a representative subsample is selected and assessed and the findings are extended to the full population. The methods used for ensuring that a sample is truly representative can be complex, but the results of an assessment can be misleading if the individuals who are assessed differ from the rest of the group in either intakes or requirements. Errors can arise if the sample is nonrepresentative. For example, a telephone survey might select more high-income participants by missing families who are too poor to own a telephone. Alternatively, the people who refuse to participate are not a random subsample (e.g., working mothers might be much more likely to refuse than retired people). Therefore, assistance from a statistician or other expert in survey sampling and design should be obtained (Dwyer, 1999; Van Staveren et al., 1994).

Determining Standard Deviations of Prevalence Estimates

Is the estimated prevalence of nutrient inadequacy in a population significantly different from zero?

Answering this question requires estimating the standard deviations assodated with the prevalence estimates.

The prevalence estimates obtained from the application of either the probability approach or the Estimated Average Requirement (EAR) cut-point method are exactly that: estimates. As such, there



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