Are age-adjusted rates usually different from unadjusted rates in NHANES data?
That depends mainly on two factors: 1) whether the two subgroups in comparison are very different in age distribution; and 2) whether the health outcomes or risk factors being compared are associated with age. If yes to both cases, you will usually find the age-adjusted rates considerably different from crude rates. This suggests that age-standardization is necessary. Nevertheless, it is generally good practice to use age-adjusted estimates when comparing health outcomes among subgroups, or at least compare the age-adjusted estimates with the crude rates to make sure there are no substantial differences, before using the crude estimates.
