There is a lot of interesting information about national/ statewide housing at:

http://www.ofheo.gov/

Here is the quaoted definition of the Housing Price Index in OFHEO's own words:

" The HPI is a broad measure of the movement of single-family house prices.  The HPI is a weighted, repeat-sales index, meaning that it measures average price changes in repeat sales or refinancings on the same properties. This information is obtained by reviewing repeat mortgage transactions on single-family properties whose mortgages have been purchased or securitized by Fannie Mae or Freddie Mac since January 1975.

The HPI serves as a timely, accurate indicator of house price trends at various geographic levels.  Because of the breadth of the sample, it provides more information than is available in other house price indexes. It also provides housing economists with an improved analytical tool that is useful for estimating changes in the rates of mortgage defaults, prepayments and housing affordability in specific geographic areas."

 For the latest OFHEO news releases go to

http://www.ofheo.gov/newsroom.aspx?ID=435&q1=1&q2=None