Data on building authorisations is obtained each quarter by postal survey of builders, owners, and other applicants. The survey is called the Quarterly Building Activity Survey (QBAS). GST and consents valued below $5,000 are excluded.
Building consents issued by councils are grouped each month into four value ranges for residential buildings and four value ranges for non residential buildings.
- Highest value range: For all consents, builders or consent applicants are surveyed to obtain values for building work put in place during the quarter.
- Second/third value range: A sample of builders or consent applicants is surveyed and the quarterly values collected are rated up so as to represent both surveyed and non-surveyed building work.
- Lowest value range: The consent values are used to represent the quarterly value of building work put in place.
Surveyed building jobs that are not completed at the end of the quarter are surveyed again in following quarters until the work is finished.
The rating up of sampled values and calculation of sampling error are complex and depend on factors that differ for each value range and month of selection. For more detailed information on the survey methodology, contact the Statistical Methods Section, Statistics New Zealand, Private Bag 4741, Christchurch.
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.
The seasonally adjusted series are re-estimated quarterly when each new quarter’s data becomes available. Figures are therefore subject to revision, with the largest changes normally occurring in the latest quarters.
The X-12-ARIMA seasonal adjustment program, developed at the U.S. Census Bureau, is used to produce the seasonally adjusted and trend estimates.
Further information about seasonal adjustment is on the Statistics NZ website (www.stats.govt.nz).
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 quarterly when each new quarter’s data becomes available. Figures are therefore subject to revision, with the largest changes normally occurring in the latest quarters. 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.
Constant price series
Current values include both a quantity and price component, whereas constant price (deflated) values have had the effect of price change removed. This leaves just the volume (or quantity) component, meaning that deflated values provide a measure of the quantity of building work being done each quarter. Comparisons among different time periods are more meaningful when there are no distortions caused by price inflation.
Quarterly values for residential building work and non-residential building work are separately deflated by the residential buildings and non-residential buildings sub-indexes from the Capital Goods Price Index (www.stats.govt.nz). The deflated quarterly values are expressed at a constant pricing level, which in this case are prices as at the September 1999 quarter. Deflated quarterly values are also seasonally adjusted and estimated trend values are calculated. Deflated values for all buildings are calculated as the sum of the deflated values for residential and non-residential buildings.
Prior to the June 2006 quarter release, price deflation was done after seasonal adjustment and estimation of trend values. Price deflation is now done before seasonal adjustment and estimation of trend values. Values for the deflated series have been recalculated for all previous quarters. In real terms, the recalculated values are generally within 1 percent of the values produced by the previous method.
Series calculated using the old method and June 1991 quarter expression base are no longer published but can be provided on request.
For more information, follow the link from the Technical notes of this release on the Statistics NZ website
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