That is based on the decrease of flow into the US, not a domestic head count.
It is based upon a comparison between previous census data and 2009 census data; RE: "according to the Pew Hispanic Center, which based its report on an analysis of 2009 census data"; this data was 'used' to extrapolate a supposed 'flow' rate that applies to now.
IF they could measure flow THEN they could stop flow. We see here some creative statistics...
Do you happen do know just what the questions pertain to answering in the affirmative as to "illegal alien status"?
I am sure that the data is quite accurate from these unofficial and partial sample counts -yes most assuredly on the up and up. One thing does bother me though -HOW exactly does one get an accurate count by sampling?