Official statistics generally measure more than one data point in time, rather than presenting a single number or percentage. This allows for the measurement of change and the identification of trends. Economic indicators present rates of change over specified periods - inflation in one month is compared with a previous month (month on month).
This is demonstrated well in yesterday's release of the latest consumer price index (CPI) data, where it was reported that the official inflation rate (the percentage change in the CPI for the historical metropolitan areas at September this year compared with that of September last year) was 5.3 percent.
This is 0.1 percentage points lower than the annual rate of 5.4 percent in August.
Even when a specific statistic is presented, such as the size of the population measured through a full population census, this is compared with one or more previous censuses and indicates change in population size.
Stats SA compares data over time. Sometimes this involves measuring the same phenomenon, such as mining sales or wholesale trade sales. In other cases, merging of data is undertaken, where information from different data sets is integrated to measure complex phenomena such as poverty, or to construct multidimensional indices measuring deprivation or human development. Estimation of gross domestic product (GDP) involves a similar integration and comparison of multiple data sources.
Stats SA updates time-series data for a set of key economic indicators such as the CPI and GDP. This allows for the long-term comparison of data so that change can be measured and trends identified.
An important initiative facilitating data comparison of labour statistics over time was completed in September last year, when a historical series of revised estimates for the labour force survey (LFS) was published. This allowed for the comparison of data from September 2000 through to March last year.
It involved updating the entire series of LFS results to reflect new population estimates. In addition, all rounds of the LFS were benchmarked to Census 2001, including the early rounds that were previously benchmarked to Census 1996. This has rendered the entire LFS series comparable.
A similar exercise was done, which facilitated comparison between data sets compiled from October household surveys conducted between 1995 and 1999. This allowed Stats SA to analyse changes in life circumstances of individuals and households in the first five years of political transition following the 1994 elections, comparing key data from the survey of 1999 with data from the 1995 to 1998 surveys.
Comparison of results of the 1995 and 2000 income and expenditure surveys painted a picture of how income is distributed in South Africa by using five income quintiles. It examined expenditure patterns in households falling into different expenditure groups.
The 1995 findings on income and expenditure, inflated to 2000 market values, formed the basis of comparison with the data collected in 2000.
A report based on these comparisons looked at other changes in poverty over time, since poverty should not be viewed only in monetary terms. As a result, findings on poverty from previous Stats SA studies could, through data comparison and confrontation, be updated.
The Pan-African Census Explorer system, currently under development, will archive population census data and metadata so that information from population censuses held in 1970, 1980, 1985, 1991, 1996 and 2001, as well as censuses held in Bophuthatswana in 1980 and 1991 and the Ciskei and Venda in 1991 can be compared.
As part of this project, Stats SA has recently used data from the 1991, 1996 and 2001 censuses to measure change in areas in Pretoria. This made necessary the creation of a common geography for Pretoria across 10 years, so that the data being compared could be attached to selected areas.
Comparison of data over time and between and across statistical series requires consistency in methodology used and data items for which information is gathered. Where these change because of advances in statistical practices, quality improvement, changes in sampling methodology or the addition of new data items, this has to be recorded in the metadata that describe the data.
A centralised repository of metadata, recording all information required to use data sets, assists data comparison greatly and the building of such a facility, will be one of the first outputs of Stats SA's Data Management and Information Delivery project.
Measurement of change and identification of trends is a central element of statistical practice and is often undertaken as part of a collaboration between users and producers of statistics. In this context, Stats SA welcomed a recent media announcement that an international foundation made funds available to the University of Cape Town to facilitate data comparison on issues such as unemployment and poverty.
Analysts and researchers at universities, in the private sector, in non-governmental organisations and elsewhere, are frequently involved in data comparison activities, using official and other statistics.
Their initiatives form an integral part of the overall system of statistics and their work is often of considerable assistance in the ongoing data comparison and analysis undertaken by the official statistics agency.