They then apportion the state population to each county. The Bureau of Biometrics method instead determines the population of each county through modeling and then sums the counties to determine the statewide population. Because the Census predetermines what the state population should be, the estimates tend to be lower than the Bureau of Biometrics. In addition, the data available for the components of change may be different. Bureau of Biometrics estimates are a year or so more current, but they do not include any data on race or ethnicity.
All data concerning the size of the population are subject to error.
This means that estimates for 1991 are probably more accurate than estimates for 1997. In addition, estimates concerning smaller populations will be affected by error much more than estimates of large populations. This compounds the instability of incidence and mortality rates for smaller populations, which also have fewer incidence cases and deaths to base the rates on. The error of closure for the 1970's, however, amounted to almost 5 million persons. For the 1980's, it was 1.5 million persons.