Data Collection
Previous methodology- Value of Building Work Put in Place to September 2014 Quarter
This data collection content is regarding high level methodological information that will not change over the life of this series.
en-NZMethodology
Data source
Values for building work put in place are obtained each quarter by a postal survey of builders or consent applicants. The survey is based on building consents data and is called the Quarterly Building Activity Survey (QBAS).
en-NZThe Value of Building Work Put in Place statistical series began in 1965, and provides the quarterly estimated value of work put in place on buildings. Information for the series is obtained from building consents and from a postal survey called the Quarterly Building Activity Survey (QBAS). The monthly Building Consents Issued series is an early economic indicator of the intention to build, and the Value of Building Work Put in Place series estimates the actual value (and volume) of construction work done each quarter on buildings.
en-NZSurvey design
Building consents are grouped each month into four value ranges for residential buildings, and four value ranges for non-residential buildings, as follows:
- Highest-value range – for all residential or non-residential consents, builders or consent applicants are surveyed to obtain quarterly values for building work put in place.
- Second- and third-value ranges – a sample of builders or consent applicants is surveyed and the quarterly values collected are rated up, 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.
| Value of building work strata boundaries from 2008 to 2014 - building consent value strata boundaries for postal survey | ||
|---|---|---|
| Starting quarter | Residential building consent value | Non-residential building consent value |
| Postal survey - full coverage | $1,200,000 + | $1,600,000 + |
| Postal survey - weighted | $310,000 < $1,200,000 | $420,000 < $1,600,000 |
| Postal survey - weighted | $45,000 < $310,000 | $80,000 - $420,000 |
| Modelled low value | $5,000 < $45,000 | $5,000 < $80,000 |
Full coverage strata 01 Residential and 05 Non-residential.
Weighted strata 02 & 03 Residential and 06 & 07 Non-residential.
Modelled low value 04 Residential and 08 Non-residential.
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 further information, contact info@stats.govt.nz.
Interpreting the data
Constant price series (volumes)
Current values include both a quantity and price component, whereas constant price series (volumes) have had the effect of price changes removed. Removal of price change (deflation) leaves just the volume (or quantity) component, enabling comparisons across different time periods without the distortion caused by price inflation (or deflation).
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. The deflated quarterly values are expressed at a constant pricing level, using September 1999 quarter prices. Deflated values for all building activity are calculated as the sum of the deflated values for residential and non-residential building activity.
Price deflation is done before seasonal adjustment and estimation of trend values.
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 recalculated 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-13ARIMA-SEATS seasonal adjustment program, developed at the U.S. Census Bureau, is used to produce the seasonally adjusted and trend estimates.
Seasonal adjustment in Statistics New Zealand has more information.
Trend series
Trend calculation 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 recalculated 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-13ARIMA-SEATS 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.
Comparison with building consent statistics
Building consent statistics provide an indication of upcoming building activity, but comparisons may be affected by variable timing and valuation differences, particularly following the Canterbury earthquakes.
en-NZThe sample frame for the Value of Building Work put in Place is a list of building consents issued by Territorial Authorities (TAs) throughout New Zealand.
Each month, the TAs supply details of new building consents they have issued. These details include the value of the consent, the type of building, the owner/ builder contact details, the building site address etc. Consents worth less than $5000 are excluded. The sample frame excludes consents for building work that is classified as "Other construction " (non-building construction), such as retaining walls, swimming pools etc.
Sample selection
The QBAS is based on a stratified sample design. New building consents are split into two groups; residential and non-residential. Each of these groups is further divided into four value groups on the basis of consent value, making a total of eight strata or value groups in total, referred to as value groups 01 to 08.
The two value groups with the highest consent values - 01 for residential buildings and 05 for non-residential buildings - are full-coverage in the sample. All consents in these value groups are in QBAS. Strata 01 and 05 also include consents for multiple building types and staged consents.
The two values groups with the lowest consent values - 04 for residential buildings and 08 for non-residential buildings - are not sample. We assume that the work on these consents was started and completed within the same quarter that the consent was issued, and that the value of work put in place is the same as the consent value.
The remaining four value groups (02, 03, 06 and 07) are sampled. The sample is selected systematically (ie 1 in every nth consent is selected) from a list of consents ordered geographically within each value group. The geographic ordering ensures that the sample is allocated across geographic areas in proportion to the number of consents in the population in that area. Sample selection is independent within each value group and the sample size is fixed, that is the sampling fraction varies with the number of consents.
Estimated share of total work put in place by strata as at (date) 01 20% 05 25% 04 and 08 together 7%
Target sample sizes by strata as at (date) 02 52 03 117 06 19 07 22
en-NZ
