Measuring Migration: The Methodology
To analyze migration, we rely on two primary data sources: tax records from the Internal Revenue Service and U.S. Census records.
Each year, the IRS tracks population shifts using the social security numbers people enter on their tax returns. If a tax return with a given social security number comes from a different county than the year before, the tax filer and all of the dependents listed on that return are considered migrants.
IRS data provides:
- the total number of returns, which tells us the number of households in a given county;
- the number of individual exemptions on each return, which tells us the population in each county; and
- total income
Using IRS data to analyze migration has some limitations. Due to privacy and disclosure policies, only significant population migrations — those involving at least 10 households moving from one specific county to another — are identified by the IRS. If, for example, residents of nine households moved from County X to County Y, the county of origin — County X — would not be identified. Those migrants’ point of origin would be included in an “other migration” category. But if residents of 10 or more households moved from County X to County Y, IRS data would specifically identify County X as the originating county.
Another limitation is that someone who files an individual return one year may file as a dependent to someone else the next year due to marriage. Late filings also have an impact. Migration data provided by the IRS captures only 95 to 98 percent of all returns filed during any given year.
Despite the fact that this data cannot account for every migrant into and out of the region, IRS data does provide us with the best estimate of migration flows — flows that can tell us a lot about the Kansas City area. This data shows that Greater Kansas City is a net in-migration metropolitan area — that is, more people move into the area than leave it. This indicates that the region is doing a good job of providing opportunity for new and existing residents.
Migration data from the Census Bureau uses IRS data as a base, but also includes annual population estimates to identify the components of population change. The Census Bureau quantifies population change both from natural growth (births minus deaths) and migration (in-migration minus out-migration). It also provides a further breakdown of migration data into internal migration — people moving about within the U.S. — and international migration.
Both data sources have their pluses and minuses. IRS data does not account for all migrants, but it does provide details about where migrants are moving to and from. Census data does not track where migrants are moving to and from, but it provides annual estimates of population growth and decline for each county and tells us how much of that growth or decline is due to natural causes and how much due to migration.
To learn more about migration into and out of the Greater Kansas City area, we have analyzed both IRS data, which is available back to 1994, and Census data, which is only available back to 2000. Close analysis of both Census and IRS data shows a strong correlation between the two — not unexpected, since the Census Bureau uses IRS data as a base for its own analysis — which allows us to approximate total Census figures back to 1994.
This report analyzes migration trends in the Greater Kansas City area, as well as in nine metropolitan areas identified as peer metros in Metro Outlook.
Peer Metros
For this study, we rely more on Census data, since it provides more accurate total counts we can use to compare Greater Kansas City’s population migration figures with that of our peer metropolitan areas. IRS data is used to look at where people who left each region migrated to and where new residents came from.
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