Nationwide house prices are mix adjusted - i.e. we track a representative house price over time rather than the simple average price. We do not use the simple average price because it is too easily influenced by a change in the mix (i.e. proportion of different property types, locations etc) of houses.
Although it remains similar to the Halifax method we substantially updated our system in 1993 following the publication of the 1991 census data. These improvements mean that our system is more robust to lower sample sizes because it better identifies and tracks our representative house price.
To find out how we create our House Price Index, view the full methodology (PDF).