11 November 2010
According to a working paper by Greg Kaplan and Sam Schulhofer-Wohl at the Federal Reserve Bank of Minneapolis, recent estimated declines in interstate migration are simply artifacts of the imputation procedure used by the Census Bureau.
The bureau uses a “hot-deck” imputation procedure to match respondents who fail to respond (called recipients) to those who actually do respond (called donors) and impute the recipient’s missing values with the donor’s observed values. For migration, the crucial questions are where the respondent lived one year ago. Before 2006, they effectively did not match on current location, even though current location is a strong predictor of past location. In 2006, they switched:
Using the most recently processed respondent as the donor to impute missing answers means that the order of processing can aect the results. Since 2006, respondents have been processed in geographic order. This ordering means that the donor usually lives near the recipient. Since long-distance migration is rare, the donor’s location one year ago is also usually close to the recipient’s current location. Thus, if the procedure imputes that the recipient moved, it usually imputes a local move. Before 2006, the order of processing was geographic but within particular samples. Therefore, on average, donors lived farther from recipients; donors’ locations one year ago were also on average farther from recipients’ current locations; and recipients were more likely to have imputed interstate moves.