Because the number of people in triangle A is approximately equal to the number in triangle B, these two groups cancel each other out, and the proportion of the population above the 45° line (inadequate intakes, shaded area of graph) is approximately equal to the proportion of the population to the left of the intake = EAR line. In other words, the proportion of the population with intakes below their requirements (from the joint distribution approach) is about the same as the proportion of the population with intakes less than the EAR, even though some of the individuals in these two groups are not the same.
Box 4-2 The EAR cut-point method—when it works
The EAR cut-point method works best (produces an almost unbiased estimate of prevalence of nutrient inadequacy) when:
If the true prevalence in the group is about 50 percent—so that the mean intake is approximately equal to the EAR—then the EAR cut-point method results in almost unbiased estimates of prevalence of inadequacy even if conditions 1 and 3 are not met (see Appendix D).1
The EAR cut-point method—when it does not work
What happens when the assumptions required for the cut-point method are not met? In the following section, examples are provided of situations in which the assumptions do not hold. The cut-point method can either underestimate or overestimate the population prevalence of inadequacy under such circumstances.