One of my colleagues was looking for disability info for a county in New York State. She went to the 2006-2008 data in the American Community Survey, but found absolutely nothing. Curious, I peaked at the 2008 data and found table B18001 and successive information that was totally skipped in the 3-year data. Why WAS that?
The Census Bureau changed the questions on disability. As the state Department of Health notes: "These changes were significant enough that 2008 disability estimates are NOT comparable with previous [ones]. To ensure that there are no comparisons, the Bureau has elected to delete the old table numbers and add new table numbers. The new tables will also contain a footnote that states that the data should not be compared to previous years’ data."
Three-year data on disability will once again be available from the 2008-2010 ACS, when Census has gathered three years of data with the new questions. The five-year data on these questions will cover 2008-2012.
I guess data users just need to be very aware of breaks in various data series.
How to Use the Data: Comparing 2006-2008 ACS 3-Year Data.