What does relative risk (risk ratio) measure?

Relative risk, also known as risk ratio, quantitatively measures the risk of a particular event (like disease occurrence or death) among individuals who have been exposed to a certain factor compared to those who have not been exposed. This metric is essential in epidemiology as it helps to determine whether exposure to a certain risk factor increases the likelihood of developing a disease.

When a relative risk of greater than 1 is observed, it indicates that the exposed group has a higher chance of experiencing the event compared to the unexposed group. Conversely, a relative risk of less than 1 suggests a protective effect of the exposure against the disease. This makes relative risk a crucial tool for risk assessment and public health decision-making.

The other options do not accurately represent the definition of relative risk. Prevalence indicators are distinct from risk ratios, as prevalence reflects how widespread a condition is at a particular time rather than the relative likelihood of its occurrence based on exposure. Evaluating the odds of disease in cases versus controls pertains to odds ratios, which are used in case-control studies. Average disease duration relates to the time aspect of illness and is not pertinent to risk measurements. Thus, option A encapsulates the correct scope of what relative risk measures.