Data for building consents is obtained each month from all territorial authorities. Values include GST and are not inflation adjusted.
From September 1989, consents below $5,000 are excluded. Under the building regulations effective from 1 January 1993, building authorisations are applied for under the building consents system administered by territorial authorities. Before this date, applications were made under the building permits system. The building consents system has wider coverage than the building permits system. The additional coverage includes some government building (particularly work on education buildings), and on-site drainage and reticulation work.
Classification of building types
A building is classified according to its main intended function. Some consents are for a building that may have more than one purpose (such as a shop/office building). Before June 1996, these consents were classified to a separate multi-purpose category. From the June 1996 month, the floor area and value of a consent for a multi-purpose building is split between each of the building's main functions. When sufficient detail cannot be obtained, the building is classified according to the predominant function of the building.
Figures for new apartments are compiled from consents that have 10 or more new attached dwelling units (flats or apartments). If there are fewer than 10 flats or apartments on a consent, they are treated as being dwellings other than apartments. Apartment numbers often show large fluctuations from month to month and, unless removed from dwelling figures, can mask underlying movements.
Some consents, particularly for large projects, are issued in stages across several months. Value data is collected at each stage but floor areas and dwelling or building counts are normally recorded at the first large stage of the project. This difference in timing can affect calculations of average prices.
Seasonally adjusted series
Seasonal adjustment removes the estimated impact of regular seasonal events, such as summer holidays and pre-Christmas purchasing, from statistical series. This makes figures for adjacent periods more comparable. To reduce distortions, the series for non-residential buildings is estimated after removal of large consent values of $25 million or more.
The seasonally adjusted series are re-estimated monthly when each new month’s data becomes available. Figures are therefore subject to revision, with the largest changes normally occurring in the latest months.
The X-12-ARIMA seasonal adjustment program, developed at the U.S. Census Bureau, is used to produce the seasonally adjusted and trend estimates.
Trend estimation removes the estimated impact of regular seasonal events and irregular short-term variation from statistical series. This reveals turning points and the underlying direction of movement over time.
The trend series are re-estimated monthly when each new month’s data becomes available. Figures are therefore subject to revision, with the largest changes normally occurring in the latest months. Revisions can be large if values are initially treated as outliers but are later found to be part of the underlying trend.
The X-12-ARIMA seasonal adjustment program is used to produce the seasonally adjusted and trend estimates. Irregular short-term variation is removed by smoothing the seasonally adjusted series using optimal weighted moving averages.
Further information on seasonal adjustment is available on the Statistics NZ website (www.stats.govt.nz).
Trading day adjustments
An aim of time series analysis is to identify movements that are due to actual changes. Seasonal adjustment is done to remove systematic calendar-related variation. Specific adjustments can be made to remove variations due to trading day differences and moving holidays, such as Easter, which are not accounted for in a standard seasonal adjustment.
Some of the apparent movement in building consent figures is due to trading day differences between months. For example, a month with four weekends will have more trading or working days than a comparable month with five weekends. This can affect monthly figures, even though there may be no difference in the length of the month or difference in the rate at which consents are issued. Trading day effects, when estimated to be statistically significant, are quantified and removed. This is trading day adjustment.
Since 1998, trading day adjustments have been made to the building consents series during the seasonal adjustment process. Since May 2004, an improved method has been used. At present, there is no adjustment to remove the effect of moving holidays such as Easter.
Trend estimates versus month-on-month comparisons
Trend estimates reveal the underlying direction of movement in statistical series. In contrast, comparisons of unadjusted data between one month and the same month in the previous year/s do not take account of data recorded for the intervening months, and are subject to one-off fluctuations. Reasons for fluctuations include changes in legislation, economic variables such as interest rates, and trading day composition of months.
For more information, see the link from the 'Technical notes' of this release on the Statistics NZ website (www.stats.govt.nz).
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