Caution should be used in analyzing the municipal population estimates just released by the U.S. Census Bureau. There are a few reasons to be careful in making comparisons. For municipalities within counties, estimates are largely based on countywide growth and historical ratios between the county and municipality. For independent cities such as Baltimore City, on the other hand, a more detailed methodology is used utilizing city-specific information. In the latest figures from the U.S. Census Bureau, the population estimates for July 1, 2011 for municipalities that are part of a surrounding county were based on allocating housing unit growth at the county level to the sub-county areas, based on proportions from the 2010 Census. In other words, no municipal-specific housing unit estimates were used to formulate the municipal population estimate.

For municipalities that are not part of counties, such as Baltimore City, population was estimated with a more comprehensive methodology that involves city-specific birth, death and migration data. In this method, the most difficult component to estimate is migration, both domestic and international, which can have a significant effect on population totals where migration is a key component of population change.

Comparisons between Washington, D.C. and Baltimore City are on more solid ground because both population estimates were based on place-specific birth, death and migration data. But making comparisons with growth in smaller cities and towns in the region, such as those highlighted in an article in the Washington Post yesterday, becomes an apples-oranges comparison due to the different estimation methods. Growth rates for Rockville (1.8 percent) and Gaithersburg (1.9 percent), for example, were the same or close to the Montgomery County growth rate (1.9 percent) from April 1, 2010 to July 1, 2011. That is what you would expect when proportionately allocating county growth to sub-county areas.

Caution should also be used when comparing these current growth rates for these smaller municipalities to their historical growth rates, since current growth rates are largely a function of what is happening at the county level.

Mark Goldstein is senior demographer for the Maryland Department of Planning.