Information directly from FHFA Questions
Although both indexes employ the same fundamental repeat-valuations approach, there are a number of data and methodology differences. Among the dissimilarities:
a. The S&P/Case-Shiller indexes only use purchase prices in index calibration, while the all-transactions HPI also includes refinance appraisals. FHFA’s purchase only series is restricted to purchase prices, as are the S&P/Case-Shiller indexes.
b. FHFA’s valuation data are derived from conforming, conventional mortgages provided by Fannie Mae and Freddie Mac. The S&P/Case-Shiller indexes use information obtained from county assessor and recorder offices.
c. The S&P/Case-Shiller indexes are value-weighted, meaning that price trends for more expensive homes have greater influence on estimated price changes than other homes. FHFA’s index weights price trends equally for all properties.
d. The geographic coverage of the indexes differs. The S&P/Case-Shiller National Home Price Index, for example, does not have valuation data from 13 states. FHFA’s U.S. index is calculated using data from all states.
For details concerning these and other differences, consult the HPI Technical Description and the S&P/Case-Shiller methodology materials