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Key Terms in Statistics

HIV Prevalence
Caption: Projections of HIV-infected in 2004 made by different organisations range from 3.8-million to 5.6-million.
Source:Dorrington RE, Bradshaw D, Johnson L, Budlender D. The Demographic Impact of HIV/AIDS in South Africa. National Indicators for 2004. Enlarge image
"HIV prevalence" refers to the estimated percentage of the adult population living with HIV at a specific time, regardless of when infection occurred. It describes the HIV trends in terms of time, place (province) and age. National level prevalence surveys are usually conducted using pregnant women attending antenatal clinics as the sample population. However, they can be conducted amongst a sample of the general population (for example, as was done in the Nelson Mandela/HSRC Survey), but also in workplaces, in specific communities, or amongst particular populations (for example sex workers, or truck drivers).

Prevalence is expressed as a percentage of a particular population — for example, "20% of women attending public sector antenatal clinics were HIV-positive".

HIV Incidence

"HIV incidence" is the number of new infections occurring over a given time among previously uninfected people. This is usually expressed as a number of a particular population — for example, it is estimated that there are 600 new infections occurring per day. HIV Incidence statistics have been recorded in the latest national household survey in South Africa ( South African National HIV Prevalence Incidence Behaviour and Communication Survey, 2005). Measuring new infections is a complex process and is usually estimated rather than being measured directly.

Estimates are usually derived from antenatal prevalence surveys, by estimating incidence using prevalence rates amongst young people (e.g. 15-19 year olds) as it is more likely that any infections in this group will have taken place quite recently. Although there are limitations to this approach, changes in HIV prevalence amongst younger age groups may reflect important new trends in the epidemic.

Confidence Interval

The term "confidence interval (CI)" is often used in HIV prevalence and behavioural surveys. Confidence intervals show how precise an estimate is. For example, the HIV prevalence of 15-19 year olds might be estimated to be 13% — but we need to know how precise that estimate is. A 95% CI shows that the level of confidence that is influenced by the number of observations of HIV infection — so for example, the sample size might have been too small to make a very accurate estimate, and what is then given is the likely range of the estimate. In the case of the example, the researchers would say that they believe that the rate is 13%, but because of the limitations of their sample, this might range from 10% to 16%, with a 95% likelihood that it is 13.

Narrower CI ranges indicate a higher level of sampling efficiency — so a CI range of 12%-14% is better than a range of 10%-16%, for example.

Sources for key terms: UNAIDS & WHO. 2002. AIDS Epidemic Update: December 2002. UNAIDS. Geneva; SA National Department of Health, 2003. National HIV and Syphilis Antenatal Sero-prevalence Survey in South Africa: 2002.
Wits Journalism Anova Health

The project is jointly managed by the Anova Health Institute and the Journalism and Media Studies Programme at the University of the Witwatersrand. The project is funded by by the Health Communication Partnership based at the Johns Hopkins Bloomberg School of Public Health Centre for Communication Programmes and the President’s Emergency Plan for AIDS relief through the United States Agency for International Development under terms of Award No. JH/HESA-02-05 and through the Anova Health Institute through PEPFAR via USAID under Award No. AID-674-A-12-00015.