Gross domestic product (GDP) estimates are compiled from a wide variety of data sets. While Statistics SA is the only institution empowered to produce official statistics, GDP information is obtained from a number of other institutions, including the department of minerals and energy, the department of agriculture, the Reserve Bank and the SA Revenue Service (Sars).
Stats SA's own monthly surveys cover industries such as manufacturing and trade, reporting their findings between six and eight weeks after the reference month.
Surveys at three-month intervals, such as the quarterly financial survey and the quarterly employment survey, are released within the following quarter. The annual economic activity survey (EAS) is the principal survey on which annual and quarterly estimates of GDP are based.
The 2004 EAS, published late in October this year, covers information for the financial years that ended between July 1 2003 and June 30 2004. As such, it reflects actual economic activity that took place in the calendar years 2002, 2003 and 2004.
As the 2004 survey got under way, improved information for previous years became available, such as audited financial statements. This sometimes provided data that differed from information initially supplied.
In some cases, Stats SA obtained additional information relating to the structure of the enterprise being sampled. This led to an improvement in the standard industrial classification of the economic unit. The result of this was the revision of the 2003 EAS as part of the finalisation of the 2004 EAS. This information is incorporated into GDP estimates for the relevant periods.
The EAS is Stats SA's most comprehensive instrument covering the economy. However, some economic sectors are not included in the EAS, and information for these sectors is gathered from other surveys and institutions.
First among these is the agriculture sector. Data on the output and value added for the agricultural industry is supplied to Stats SA on a quarterly basis by the department of agriculture. These data sets are revised as more information becomes available. In addition, Stats SA and the department conducted a census of commercial agriculture in 2002, which will be followed by an annual series from 2005 onwards.
A second sector is mining. The department of minerals and energy supplies data on mining production and sales on a monthly basis to Stats SA. From this data, Stats SA calculates indices of the physical volume of mining production and seasonal adjustments on production and sales.
Another sector not covered in the EAS involves financial intermediaries. In this instance, the estimates of GDP rely on information collected by the Reserve Bank, which is provided on a quarterly basis and revised as new information becomes available.
Stats SA relies on the Reserve Bank's balance of payments estimates to take foreign trade into account when estimating GDP. Balance of payments data are closely tied in with foreign trade data collected by Sars. Again, these estimates are revised as new information becomes available.
Lastly, Stats SA has various series that collect data on the activities of government's three tiers. Provincial and central government data is derived from audited financial statements, often only available many months after the reference period. Until then, proxy indicators are used to estimate quarterly GDP.
Similarly, Stats SA has surveys that collect municipal data, which also influence measurement of industries such as electricity, gas and water, and transport, storage and communication.
Many of the series that feed into GDP estimates are revised as a natural part of the collection of data, and the analysis and reporting of economic growth. The publication of GDP estimates on November 29 will include revised annual and regional estimates for 2002 to 2004 as well as quarterly estimates up to the third quarter of 2005.