FIGURE 4-6 Risk curve combined with a usual intake distribution where mean intake (115 units) is slightly higher than the Estimated Average Requirement (EAR) (100 units). The risk curve and usual intake distribution have significant overlap. The proportion of individuals at risk of inadequacy (shaded area) at the mean intake is about 25 percent. The risk of inadequacy increases as intake becomes closer to the EAR.

Determining the prevalence of inadequate intakes for the population will depend on how many people have each particular value of intake and what the distribution of intakes looks like. Appendix C demonstrates how to carry out the necessary calculations to obtain a prevalence estimate for the group. Statistical programs (such as SAS or similar software) can be used to carry out these procedures.

Two key assumptions underlie the probability approach: (1) intakes and requirements are independent, and (2) the distribution of requirements is known. Frequently, it is assumed that the distribution of requirements is normal; however for some nutrients, such as iron for menstruating women, this assumption is not warranted (some women have very large menstrual losses of iron, which leads to a distribution that is positively skewed—i.e., more women have higher requirements than indicated by a normal distribution). For other nutrients the numbers of people for whom requirements have been experimentally determined is so small that it is just not possible to determine whether the assumption of normality is warranted (IOM, 1997, 1998b, 2000; NRC, 1986, 1989).