Studies observing people (sometimes called population, observational or epidemiological studies)

Epidemiologists (a kind of scientist) study how often diseases occur in different groups of people, and why. Their studies on populations will often report an association between a substance or behaviour and a disease, and this will be described by journalists as a ‘link’ – ‘red wine linked to cancer’ for example.

Epidemiological studies often involve large numbers of people and they can provide important insights into the causes of disease; it was an epidemiologist who first suggested smoking was linked to lung cancer.

However, although they might flag up associations, this kind of study can’t establish whether the substance or behaviour in question is the direct cause of a disease. This is because epidemiological studies monitor the health of populations in real-life, uncontrolled conditions, where all sorts of other things including lifestyle, diet or access to healthcare might also influence someone’s health.

Controlling for these other factors is very difficult. To demonstrate a causal link, more controlled studies (for example randomised controlled trials) would be needed to follow up the observational study.