The Halifax House Price Index is the UK's longest running monthly house price series with data covering the whole country going back to January 1983. The Index is based on the largest monthly sample of mortgage data, typically covering around 15,000 house purchases per month, and covers the whole calendar month. From this data, a "standardised" house price is calculated and property price movements on a like-for-like basis (including seasonal adjustments) are analysed over time. Properties over ?1 million are included and the index is seasonally adjusted with the seasonal factors updated monthly The standardised index is seasonally adjusted using the U.S. Bureau of the Census XII moving-average method based on a rolling 84-month series. Each month, the seasonally adjusted figure for the same month a year ago and last month's figure are subject to revision.