Problematic Imputation at the Census 

 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 &#8220;hot-deck&#8221; imputation procedure to match respondents who fail to respond (called recipients) to those who actually do respond (called donors) and impute the recipient&#8217;s missing values with the donor&#8217;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 aect 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&#8217;s location one year ago is also usually close to the recipient&#8217;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&#8217; locations one year ago were also on average farther from recipients&#8217; current locations; and recipients were more likely to have imputed interstate moves. 
 

 (via  Gelman )