the sample age range. This step therefore, standardizes all intakes to values that would have been observed had all sampled individuals differed only in the FSP participation status. If age were the only other covariate, the standardized predicted intakes would be calculated as:
Standardized predicted intake = observed intake (Y) − b1(age − 35),
where b1 is the estimated regression coefficient associated with age.
If age is the only covariate (other than FSP participation) believed to be associated with intake, the standard predicted intakes above would correspond to intake values adjusted to a standard age (in this case 35). In essence, step 2 removes the effect of the covariates other than FSP participation on intakes. If the effect of age is to increase intake (i.e., if b1 is positive), then the standard predicted intakes for individuals who are younger than 35 will be larger than the observed intake for those individuals. On the other hand, the standard predicted intakes for individuals who are older than 35 will be smaller than the intakes observed.
Step 3. Next, the effect of day-to-day variability is removed from the standardized predicted intakes to produce an adjusted usual intake distribution. This step, described previously in Chapter 4, would be done separately for the two groups. Once an adjusted usual intake distribution has been obtained (standardized, for example, to age 35) for each group of individuals, the proportion of each group with intakes below the EAR can then be determined and compared using a simple t-test.
It is important to note that:
The estimates of prevalence of inadequacy in each of the two groups obtained using the adjusted standardized intakes will be biased, and perhaps severely so. This is because the adjusted standardized intakes have a variability that is too small. When using the standardized intakes in the adjustment procedure, one proceeds as if the regression coefficient b1 was a known, fixed value. In reality, b1 is an estimate, and as such has a variance that is not “added” to the variance of observed intakes. However, the difference between the prevalence estimates for the two groups will still be approximately unbiased, as long as the distribution of ages among the two participation groups is approximately similar, or as long as individuals in one group tend to be younger than individuals in the other group. If, however, all individuals in one group have ages clustered around