In this module, you will generate age-adjusted prevalence rates and standard errors for high blood pressure (HBP) in persons 20 years and older in the United States by sex and race/ethnicity. An optional second example is available demonstrating how to generate age-adjusted means and standard errors for Body Mass Index (BMI) in persons 20 years and older in the United States by sex and race/ethnicity.
To calculate age-adjusted prevalence rates, you will need to know the age standardizing proportions that you want to use, and then apply them to the populations under comparison. This is called the direct method for age standardization. Typically, Census data are used as the standard population structure. For age standardization in NHANES, the National Center for Health Statistics (NCHS) recommends using the 2000 Census population. A spreadsheet with the year 2000 U.S. population structure by age is attached below. Calculate the standard age proportions by dividing the age-specific Census population (P) by the total Census population number (T). The standardizing proportions (P/T) should sum to 1 (see the table below in Step 2 for the standard age proportions used in this module.)
Remember that you need to define the SVYSET before using the SVY series of commands. The general format of this command is below:
svyset [w=weightvar], psu(psuvar) strata(stratavar) vce(variance method)
To define the survey design variables for your high blood pressure analysis, use the weight variable for 4 years of MEC data (wtmec4yr), the PSU variable (sdmvpsu), and strata variable (sdmvstra) .The vce option specifies the method for calculating the variance and the default is "linearized" which is Taylor linearization. Here is the svyset command for 4 years of MEC data:
svyset [w= wtmec4yr], psu(sdmvpsu) strata(sdmvstra) vce(linearized)
For age standardization in NHANES, NCHS recommends using the 2000 Census population. To get the correct Census age distribution, you need to know two things: the age group of interest (e.g. all ages, ages 6 and older, adults 20 and older) and how wide the age strata are for adjustment (e.g. 5 year, 10 year or 20 year age intervals). In general, the more tightly you want to control for age, the narrower the age strata should be.