My understanding from reading the documentation is that in 2011, the ACS begins imputing persons in group quarters (i.e. imputing entire person records, not just a few variables). Is there a flag available to distinguish these records from the real persons interviewed?
You’re correct that from 2011-onwards, the ACS Public Use Microdata Sample (PUMS) adds synthetic records for persons in group quarters as part of Group Quarters Small Area Estimation (GQSAE). This is confirmed by the 2011 PUMS accuracy document which states that “the GQ population sample has been supplemented by a large-scale whole person imputation into not-in-sample GQ facilities.” This increase approximately doubled the number of group quarters records. I am not aware of any methods to identify these cases in the data. You can find more information about the approach used and an evaluation of its results in this 2013 Census Bureau working paper.