The concept of risk and fundamental measures of disease frequency: Incidence and prevalence

In epidemiology, risk refers to the likelihood, or in statistical language probability, of an individual in a defined population developing a disease or other adverse health problem. The prime measures of disease frequency, including probability of outcomes, in epidemiology are incidence rates and prevalence proportions. The incidence rate is the number of new cases in relation to a population, time, and place. Prevalence proportion measures all disease or a risk factor in a population, either at a particular time (point prevalence) or over a time period (period prevalence, lifetime prevalence). Rates and proportions are most accurately presented by age and sex groups (‘specific’ rates and proportions), but for ease of interpretation they may be grouped as overall, actual (crude) rates. The collection of both disease, risk factor and population data to achieve accurate figures of incidence rates and prevalence proportions is problematic, and remains a major challenge.