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 is updated as additional mortgages are purchased or securitized
by Fannie Mae and Freddie Mac. The new mortgage acquisitions are used to identify repeat transactions for the most recent period and for each subsequent period since 1975.
House price index reports are released on a monthly basis for the United States and regions and on a quarterly basis for a variety of other geographies. Most statistics in the reports reference price changes computed by FHFA's standard "purchase-only" HPI. In some cases, however, the reported statistics reference alternative price measures. FHFA publishes - and makes available for download - several additional house price indexes beyond the standard "purchase-only" series. Although they use the same general methodology, the three alternatives rely on slightly different datasets as follows:
- "All-Transactions" house price index. Appraisal values from refinance mortgages are added to the purchase-only data sample.
- "Expanded-Data" house price index. Sales price information sourced from county recorder offices and from FHA-backed mortgages are added to the purchase-only data sample. This index is used annually to adjust the maximum conforming loan limits, which dictate the dollar amount of loans that can be
acquired by Fannie Mae and Freddie Mac.
- "Distress-Free" house price index. Sales of bank-owned properties and short sales are removed from the purchase-only dataset prior to estimation of the index.
Data constraints preclude the production of all types of indexes for every geographic area, but multiple index types are generally available. For individual states, for instance, three types of indexes are available. The various indexes tend to correlate closely over the long-term, but short-term differences can be significant.