Target Population: All ready-mixed concrete producers in New Zealand.

Population Covered: The survey is full coverage of the target population.

Statistical Unit: Ready-mixed concrete locations

Selection Unit: Ready-mixed concrete locations

Collection Unit: Ready-mixed concrete locations

Population and Sample Size: The population is about 120 geographic units.

Changes in the September 2018 quarter

Seasonally adjusted and trend series were introduced in the September 2018 quarter, along with new time series.

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.

We recalculate the seasonally adjusted series 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.

We use the X-13ARIMA-SEATS seasonal adjustment program, developed in the U.S, Census Bureau, to produce the seasonally adjusted and trend estimates.

When the COVID-19 pandemic hit in 2020, we manually specially treated unusual data (outliers) to prevent them from impacting the seasonally adjusted series. For total ready-mixed concrete production, we applied additive outliers (AO) to two quarters: June 2020 and September 2021.

In the March 2023 quarter, we replaced manual with automatic AO from March 2020 quarter onwards, using less cautious criteria. For total ready-mixed concrete production, the June 2020 and September 2021 quarters remain as outliers under automatic AO detection, with minimal change to the seasonally adjusted series. We also removed trading-day adjustments as they were unnecessary.

Seasonal adjustment and automatic outliers in time series after COVID-19 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.

We recalculate trend series 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.

We use the X-13ARIMA-SEATS seasonal adjustment program 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.

Seasonal adjustment at Statistics New Zealand has more information.