Background Notes

Definition

The Residential Property Price Index (RPPI) is designed to measure the change in the average level of prices paid for residential properties sold in Ireland. The index is mix-adjusted to allow for the fact that different types of property are sold in different periods.

Data Source

The RPPI is compiled using data on mortgage drawdowns provided on a monthly basis by 8 of the main Mortgage Lending Institutions under Section 13 of the Housing Act (2002). This data provides details on the characteristics of properties bought (such as building type and size) as well as the price paid. It is transactions based; meaning that prices are recorded only where a sale occurs. Not all residential property transactions are funded by a mortgage (i.e. they are cash based) and these transactions are excluded from the scope of the index.

The CSO is currently examining Stamp Duty returns to the Revenue Commissioners, made via the Revenue Online Service (ROS), with a view to assessing both the extent of cash-based full market price transactions and any potential bias in the RPPI that might accrue from their exclusion.

Mix Adjustment

Residential properties are heterogeneous, meaning that no two houses or apartments are exactly identical. This poses a challenge when trying to construct a price index as there is a need to separate pure price change from differences in the quality of the products being bought over time. Typically this is done by comparing the prices of exactly the same products, time after time. This is, for example, the method used in the Consumer Price Index. However, in the case of residential properties, price is determined by many characteristics (location, size, build type etc) which make direct price comparisons difficult. Furthermore, only a small portion of the total housing stock is sold in any given month. The combination of these factors means that the matching process that would typically be used to calculate a standard or typical price index cannot be used in the case of houses and apartments.

The hedonic method is the prevalent statistical process for the measurement of residential price change. In this method, a number of characteristics which influence prices are analysed so that we can estimate and exclude the part of the price change that can be attributed to them. These characteristics are; location, building type, floor area, number of bedrooms, new or old and first time buyer or not. By excluding the price change determined by these characteristics we are left with an index of pure price change for a consistent set of characteristics - or more simply - a residential property price index. This index uses the rolling year hedonic regression model.

Weights

Weights are calculated at the beginning of each year based on the value of transactions during the previous year as given by the mortgage drawdown data. The index is an annual chain-linked Laspeyres-type index. It is calculated by updating the previous month’s weights by the estimated monthly changes in their average prices.

Periodicity

The index is compiled on a monthly basis. In order to smooth out short-term volatility in the series and highlight longer-term trends the published indices are based on a 3 month rolling average, i.e. a simple average of the current month and the previous 2 months. However, care should still be taken when interpreting monthly figures which may indicate short-term volatility rather than underlying change in longer term price trends.

Aggregate indices (National - all residential properties, National - houses, National - apartments, National excluding Dublin - all residential properties and Dublin - all residential properties) are compiled using monthly indices rather than the published 3 month rolling average indices . As a result these higher level indices may, in some periods, show a slight variation with their published constituent indices.

Geographic Split

The indices for Dublin include residential properties in the 4 local authority areas, covering both Dublin city and county.

Calculating percentage changes in the index The movement of the RPPI is expressed as percentage change, rather than a change in index points, because index point changes are affected by the level of the index in relation to its base period, whereas percentage changes are not.

The example below illustrates the computation of a percentage change: