Written by: Director Robert Groves
The point of the first three posts on census quality was to note that there is no known “truth” with regard to the population of the United States. We have different tools, each of which gives us a different look at the population at different points in time, but each of which has some weakness.
This post is a short description of the method of “demographic analysis.” It provides estimates of population counts at the national level only. It provides separate estimates by single years of age, separately for males and females, separately for two race groups (Black and nonblack).
It does not provide separate estimates for each state (and thus is not helpful in providing a different set of reapportionment figures); it does not provide estimates for the 8 million different census blocks (and thus does not permit different redistricting estimates).
Just like the census, demographic analysis has a simple ideal. If, for every year, we could count all the births in the country, subtract all the deaths, add all immigrants to the country, subtract all emigrants, then we could have a perfect accounting of the total population and how it has changed across time, and from one census to the next.
If we could know the gender of births, and the age and gender of deaths, immigrants, and emigrants, we could produce counts by males and females separately by age. If we could do the same for race groups, we could produce counts by different race groups.
This is essentially what we try to do, but, as with the census, we can’t achieve the ideal. Each of the major components of demographic analysis has some problem in practice and need a fix:
1) Because of the quality of the historical data on births, for the age groups 65 years and older, we use counts from Medicare enrollment by age, gender, and race.
a. Problem: not all those 65 and older are enrolled in Medicare. Fix: use survey data on medical insurance coverage to estimate the number not enrolled.
b. Problem: Medicare uses different race measurement than Census. Fix: Supplement the Medicare data with data from other sources and in some cases assume race measurement is equivalent.
2) For those under 65 years of age:
a. Problem: underegistration of births in older age groups (e.g., 6% of Black births in 1950). Fix: use estimates of the completeness of birth registration from birth registration research, combined with professional judgment, to adjust birth counts
b. Problem: different race measurement on birth and death records than on Census. Fix: examine patterns of racial identification in the last census and use professional judgment to assign groups only to “black” and “nonblack” race categories.
c. Problem: no direct count of components of international migration. Fix: use a combination of survey and other data sources and professional judgment to estimate international migration
Every component that involves some professional judgment is subject to professional debate. Some of the debates concern very small categories, which do not individually affect the national figures too much.
Increasingly, the component generating the most controversy is the count of international migrants by race, age, and gender groups. There is little consensus among demographers about how large those groups are.
Most professionals believe that the strongest features of demographic analysis come from its consistency over different age by gender groups. When the estimates of these group sizes systematically vary from the counts based on a census, many believe that is evidence of weaknesses in the census, not in the demographic analysis. Few professionals would attempt to adjust the census counts based on these comparisons, however, given the problems reviewed above.
To be transparent with the American public, in early December 2010, we will release a preliminary range of estimates by age, gender, and race, developed using the demographic methods above. This range of estimates can be compared to the total population count we release in late December from the 2010 Census.
Later, in the Spring of 2011, the range of estimates by age, race, and gender can be compared to the 2010 Census counts. Then, we may be able to improve our assumptions about components to the demographic analysis estimates, and then release an improved set of demographic analysis numbers sometime in 2011.
I expect in late December, when we release the 2010 Census counts and compare them to the demographic analysis range of estimates, journalists will ask which one is correct. As you can see from the description above, I won’t be able to answer that question. Each method has strengths and weaknesses for the various statistics of interest.
Please submit any questions pertaining to this post to ask.census.gov