The Consumer Price Index (CPI) is probably, rightly or wrongly,
the most commonly used statistic in the calculation of
The percentage movements in the CPI are used in the
- various social welfare benefits provided by the government, eg
basic family payments
- government excise duties, eg on beer, cigarettes and
- business contracts of various types, eg building contracts and
(Indexation is the process whereby payments are increased so
their value keeps pace with inflation.)
What is the CPI?
The CPI is a fixed weighted price index that relates to
household expenditure on retail goods and services and other items
such as housing, government charges and consumer credit charges. (A
fixed weighted price index is one that measures the changes over
time in the purchase price of an unchanging basket of
goods and services - see below.)
Because it is a price index the CPI measures price movements
from a reference base period (currently financial year 1989-90)
which is assigned the value 100.0. The CPI does not measure price
levels in dollars and cents.
Scope of the CPI
The scope of the CPI is capital city households where the
members of the households are employees who obtain at least 75% of
their income from wages and salaries. The top 10% of households,
based on income, are excluded.
The CPI target households account for about 60% of the
population of the eight capital cities and more than one third of
the population of Australia.
Coverage of the CPI
The coverage of the CPI is the goods and services purchased by
the capital city, wage and salary earner households (described
Expenditure patterns are determined for the CPI households
predominantly from the Household Expenditure Survey (HES) that is
run approximately every 5 years by the ABS. From the HES a basket
of typical goods and services is determined and each item within
the basket is given a weighting dependent upon its relative
importance in the expenditure patterns of the target
Each quarter around 100,000 price quotations are collected to
calculate the CPI. (Sometimes prices need adjusting to correct for
changes in the quality of goods and services.) These prices are
then sorted into 8 expenditure groups (food; clothing; housing;
household equipment and operation; transportation; tobacco and
alcoholic drinks; health and personal care; and recreation and
education) which make up the basket of goods and services.
Since the CPI is a fixed weighted index it becomes increasingly
outdated as time passes because the mix of goods and services
purchased by the CPI households changes as prices change, as new
products are introduced and as other products are no longer
available. Therefore, the basket of goods and services is usually
updated about every 5 years, when the results of the latest HES can
be incorporated, to reflect the households' changing expenditure
A new CPI series is thus formed and is linked to earlier CPI
series to form a chain of CPI series. The linking procedure is done
in such a way that the differences in the prices of the old and new
baskets have no effect on the index series.
The current CPI is the twelfth series that has been linked to
form a continuous series from the September quarter 1948.
All Groups Index
The 8 expenditure groups for each of the capital cities are
combined into the All Groups, weighted average of the
eight capital cities index and this is the index that is
generally referred to as the CPI. (More recently the percentage
change in the All Groups index has also been called the
headline rate of inflation to differentiate it from the
underlying rate that is also calculated from the CPI and
used in economic analysis.)
Calculation of Price Changes
The percentage change in prices between any two periods can be
calculated from the CPI using the following formula:
MSB Table 2.2
Monthly Economic and Social Indicators Table
2.2 tabulates the All Groups, weighted average of the eight
capital cities index along with the quarterly percentage change (ie
change from the previous quarter) and annual percentage change (ie
change from the same quarter of the previous year) in the
The annual percentage changes in the index are also presented in
The CPI is a quarterly series that is updated in the 4th week
after the end of the quarter.
Further information can be obtained by contacting a member of
the Statistics Group of the Parliamentary Research Service.
This feature was prepared by Stephen Barber.
Back to top