f. If only 3-year or 5-year period estimates are available or sufficiently precise, use them with care for analyzing trends over time for an area or population group. In general, avoid analyses of changes over time that are based on overlapping period estimates (for example, 5-year period estimates for 2010–2014 and 2011–2015).
It is not straightforward to interpret the meaning of differences that are observed between pairs of 3-year or 5-year period estimates: an observed difference may reflect a gradual change over the period, or it may reflect another pattern of change, such as stability in a characteristic followed by a sudden increase or decrease. Examining 1-year (or 3-year) period estimates for a larger area may help determine the appropriate interpretation of differences that are observed between pairs of 5-year period estimates for smaller areas within the larger area.
The less that pairs of 5-year (or 3-year) period estimates overlap in time, the more precise will be an estimate of differences between them—for example, a difference observed between estimates for 2010–2014 and 2015–2019 will be more precise than a difference observed between estimates for 2010–2014 and 2011–2015. Indeed, to obtain an acceptable level of precision for analysis of population groups, it will generally be necessary to wait for a second, nonoverlapping estimate to become available to compare to an earlier estimate.
g. Take advantage of the availability of 1-year and 3-year period estimates for PUMAs, which include about 100,000 people, to assist with analyses for smaller areas.
As one example, 5-year period estimates for small areas (census tracts in a city, towns in a county, small counties in a state) could be updated by adjusting their 5-year period estimates to the latest 1-year (or 3-year) period estimates for the applicable PUMA, as in Section B.2 above. Such adjustments need to be performed with care.
As another example, 1-year period estimates for large cities or counties could be compared with the PUMA estimates for the rest of the state (or the rest of the county in the case of a large city within a very large county).
h. Take care to label ACS estimates, including those for 1 year, 3 years, and 5 years, as period estimates.