The Recommended Dietary Allowances serve two basic uses: As guidelines for planning diets and food supplies and as a tool for evaluating nutritional adequacy of food consumed. Establishing standards to serve these functions is not simple. Differences in individuals in different population groups dictate allowances with relatively high margins of safety to prevent deficiencies. Even so, anomalies arise. Surveys have shown that intakes of vitamin A and protein are likely to exceed their respective allowances. On the other hand, calcium and thiamin intakes of older women and iron in teenage boys and women in the reproductive years are difficult to meet in terms of nutrient density ratios. In addition, insufficient data make it impossible to set allowances for some essential trace elements--and, other essential elements may yet be discovered. Thus the allowances are not guaranteed to represent the totality of nutritional needs.