Although both indexes employ the same fundamental repeat-valuations approach, there are a number of data and methodology differences. Among the dissimilarities:
a. The Case-Shiller Indexes® only use purchase prices in index calibration, while the all-transactions HPI also includes refinance appraisals. FHFA's purchaseonly series is restricted to purchase prices.
b. FHFA's valuation data are derived from conforming mortgages provided by Fannie Mae and Freddie Mac. The Case-Shiller Indexes use information obtained from county assessor and recorder offices.
c. The 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 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 on these and other differences, consult the HPI Technical Description (see http://go.usa.gov/8BBT) and the Case-Shiller methodology materials (see http://us.spindices.com/documents/methodologies/methodology-sp-cs-home-priceindices.pdf
A paper that analyzes in detail the methodological and data differences between the two price metrics can be accessed at http://go.usa.gov/8BBT