In direct adjustment, the standardized population may be:

• A. All of the above
• B. A combination of the two populations
• C. One of the two populations
• D. A completely outside population

Answer: (D)

Direct adjustment uses a fixed reference population to apply the observed age-specific rates from each group being compared, producing a rate that is not confounded by different age structures. The reference population is a choice you make; it doesn’t have to come from the groups you’re comparing. It can be an external population, it can be formed by combining the two populations into a synthetic standard, or it can be one of the populations itself if that’s chosen as the reference. Because of this flexibility, the standardized population may be a completely outside population, a combination of the two populations, or one of the two populations.