ACS Community Indicators MSA Definition Alignment

Emsi’s ACS Community Indicators data comes from the Census’s American Community Survey (ACS) API. Emsi uses the Census Bureau’s latest MSA definitions, but ACS occasionally lags, using older MSA definitions. Emsi converts the ACS data to current Census Bureau MSAs using a few different methods depending on the data type.

Many of the measures published in the dataset are aggregate measures, meaning that we can derive the value for a larger area by adding up the values of its component smaller areas. For instance, we can calculate the total population for an MSA by adding up the population of each county within that MSA (MSAs are groups of counties). For these measures, we can easily find the value for the Emsi MSA, since we can just use the ACS data published at the county level to aggregate up to the Emsi MSA.

Other measures can’t be directly aggregated from the component counties, but they can be calculated based on other data which can be aggregated. For example, we can find what percentage of the population of an MSA is disabled by finding the total disabled population and dividing it by the total population (both of those values are found by aggregating from the county data). Many of our percentages are calculated this way.

Finally, there are some values that can’t be aggregated from the counties, nor can they be derived from other measures. For example, there’s no reasonable way to calculate measures like median income or median monthly rent using data from the component counties, since medians can’t be summed and averaging them is not recommended. For cases like these, we take the value for the ACS MSA that comprises the largest portion of the population of the Emsi MSA and apply that value to the Emsi MSA.

By using these three methods, we’re able to publish ACS data that’s modeled to match our Emsi geographies. Since Emsi uses Census Bureau MSA definitions, these measures are temporary for any given MSA definition change until ACS data catches up with the change. Where ACS and Emsi MSA definitions match, ACS and Emsi data will match.