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The Census Bureau publishes the ACS sampling error with every estimate, expressed as the margin of error with 90 percent confidence. The following is an example:

The estimate of the poverty rate in Maricopa County in 2009 was 15.2 percent, with a margin of error of + or – 0.6 percentage points. The interpretation is that there is a 90 percent likelihood that the actual poverty rate was within the confidence interval of 14.6 percent to 15.8 percent. A one-in-ten chance exists that the real rate was outside this range.

For the nation and other very populous areas, the annual ACS estimates are highly accurate. For moderately populous areas such as Arizona, sampling error is moderately large, so caution is urged in using the annual ACS data for the state. The sampling error for less populous areas, such as most of Arizona’s counties, is quite large, even when using five-year averages. Thus, considerable caution is urged in using the five-year ACS data for counties other than Maricopa and Pima.

The published margin of error should be taken into consideration when deciding whether to use a result from the ACS. Whether the reported sampling error is too large depends on the user’s purpose for accessing the data and on the volatility of the measure over time and across geographic areas.