If more individuals in the group that was exposed to the campaign used condoms, the campaign was successful. If the two groups did not differ, the campaign is ineffective. Experiments have the advantage that they provide easily interpretable results because only the experimentally manipulated variable can explain differences between two otherwise identical groups. However, a drawback of experiments is that many important variables cannot be manipulated experimentally. For example, it is not possible to experimentally assign people to a group of ecstasy users and non ecstasy-users to study the health consequences of taking ecstasy.
Correlational studies take advantage of natural variation among individuals.
They examine relations between two characteristics of individuals across different individuals, which may reveal a causal relationship between the two variables. For example, before the HIV virus was discovered, doctors noticed a correlation between homosexuality and AIDS symptoms. A correlation does not tell us anything directly about a cause. It is possible that AIDS symptoms cause homosexuality. However, homosexuality has been around for a long time and was not associated with AIDS symptoms.