This measure has two major limitations, the first being that people often do not know the exact age of their sex partners. This is more likely to be true of casual partners than of spouses. The second is that the age difference constituting an elevated risk of exposure to HIV is not precisely known.
When uncertain about a partner’s age, respondents frequently give numbers clustering around numbers such as 20 or 30. This may well distort the indicator. It should be noted, however, that the biases introduced through age clustering or age misreporting are unlikely to change greatly over time, so this may be of little consequence when trends are being examined.
This measure cannot give an exact picture of patterns of age-mixing and cannot capture small shifts in the age gap between partners. Nevertheless, it should capture the substantial changes in age-mixing promoted by programmes on HIV prevention and life skills, since women are unlikely to mistake a peer for a man much older than themselves. If women increasingly choose to have sex with their peers rather than with older men, or if older men become less likely to seek out substantially younger partners, these changes will be reflected in the indicator, regardless of errors in age-reporting.