Yes, they are voluntary, and they benefit from the terrific advantage of having a benchmark like the census against which they can be compared. So we know exactly, when we design the survey and when the data become available, how they compare on those variables that are included in both surveys--how they compare with the census and whether or not some potentially very big biases have crept in that need to be corrected.
On July 27th, 2010. See this statement in context.