In addition to the basic strategies outlined above, in the future it may be possible to extract more value from the ACS 5-year period estimates by linking them with administrative records and other sources of local-area information (see Section 7-D.1).
d. When using ACS data to estimate shares of some total, compare estimates among areas or population groups, or assess trends over time, use ACS estimates that pertain to the same time period (1-year, 3-year, or 5-year) for all geographic areas or population groups that are being compared. Do not use a mixture of different period estimates.
For example, when determining the share of federal or state program funds that is to be allocated to each county in a state, the ACS estimates that are used will most likely need to be 5-year period estimates. The reason is that 1-year and 3-year period estimates are available for only about one-fourth of counties (refer back to Table 2-5), and it is not equitable to use a mixture of 1-year, 3-year, and 5-year period estimates to determine each county’s share of funds.
An exception to the need to use 5-year period estimates for fund allocations to counties is when a state has only a small number of counties that lack 1-year (or 3-year) period estimates. In this case, it may be appropriate to update the 5-year period estimates for the smaller counties by using information for larger areas, so that the equivalent of 1-year (or 3-year) period estimates can be used for all counties in the state. A simple procedure for accomplishing the updating is described in Section B.2 above; the use of this or another procedure depends on the reasonableness of the underlying assumptions.
As a matter of good practice, differences that are observed in comparing areas or population groups or in assessing trends over time should be evaluated not only for statistical significance, but also for substantive importance—that is, whether the differences are large enough to matter for policy, planning, or research purposes.
e. When analyzing trends over time for an area or population group, use ACS 1-year period estimates whenever they are available and sufficiently precise for the purpose of interest and be cognizant of changes in geographic area boundaries that may affect comparability. Keep in mind that the sampling error for the estimate of the difference between pairs of 1-year period estimates will be larger than the sampling error of either estimate.