As discussed in the Highlights article accompanying the 2011Q1 HPI Release (available for download at http://go.usa.gov/8k5d
), the census division indexes are constructed from statistics for the component states. For the quarterly all-transactions and purchase-only indexes, the census division indexes are constructed from quarterly growth rate estimates for the underlying state indexes. Census division index estimates are "built-up" from quarterly growth rate estimates (monthly growth rates for the monthly index) for the component states.
The census division indexes are set equal to 100 in the relevant base periods. Then, the index values for subsequent periods are increased (or decreased) by the weighted average quarterly (or monthly) price change for the underlying states. Index values for periods before the base period are calculated in a similar fashion; beginning with the base period value, the preceding index values are sequentially determined so that the growth rate in each period always reflects the weighted average growth rate for the component states.
The national HPI is constructed in an analogous fashion, except that the weighted components are census divisions. Because the census divisions measures are themselves weighted averages of state metrics, the U.S. index is equivalent to a stateweighted metric.