Section 2.6 of this report pointed out that the maximum number of years for which combined data would be meaningful depended on the specific item. Most health related items and fertility patterns change rather slowly over time, and the most recent 3 to 5-year averages will generally reflect current conditions reasonably well. In fact, the NHIS has published 3-year average data for Asian and Pacific Islanders (as a combined group), so a precedent exists. Economic statistics, however, are likely to be much more volatile; thus the time period should be considerably shorter. (However, in the absence of any other data, even somewhat outdated information such as a 3-year average, will be better than relying on the decennial census as the source of information for the full intercensal period. It is interesting to note that the ACS is planning to combine up to 5 years of data in order to produce reliable, small area data.)
To provide the greatest flexibility for users of this report, we will examine the improvement in precision for three combinations of years 2, 3, and 5 years.