|Unit||Index 2017Q2=1000, NSA|
|Adjustments||Not Seasonally Adjusted|
The consumers price index (CPI) measures the changing price of a fixed basket of goods and services purchased by New Zealand households. The selection and relative importance of the goods and services in the CPI basket represents the overall expenditure pattern of New Zealand households. Quarterly from 1999.
The aim of the CPI is to measure price changes of the same sample of products at each outlet over time. When there is a change in the size or quality of any of the goods or services in the basket, we make an adjustment to ensure that the price change shown in the CPI is not affected by the change in size or quality.
Often used as a measure of inflation, it covers prices for:
CPI is collected through three main methods: visiting retail outlets, postal surveys, and the Internet.
Since the 2017 review of the CPI, we have used the geometric mean, or Jevons, formula to calculate the elementary aggregate indexes for items where outlet substitution is possible (eg for groceries and appliances
We use the 'ratio of arithmetic mean prices', or Dutot, formula for items where outlet substitution is not possible (eg local authority rates), where prices are subsidised and may fall to zero (eg GPs' fees), for fresh fruit and vegetables (as the first stage of aggregation is across both outlets within each region, and across weeks within each month), and where it is not currently practical to adopt the Jevons formula (eg when prices are aggregated directly to a national elementary aggregate, rather than aggregated to a regional level).
The CPI is produced quarterly from prices gathered in a range of surveys at 15 urban areas. Its base (=1000) is currently the June 2017 quarter.
Reviews of the CPI are undertaken every three years.