Measures of disease occurrence

This chapter provides a brief introduction to some of the most common measures of disease occurrence used in epidemiology, both the empirical and theoretical versions of the measures. It begins with the prevalence of a disease in a population, which is the fraction of the population that has the disease at a given date. The chapter then considers mortality rate, incidence rate, standardized mortality ratio (SMR), and survival. Mortality is typically reported as a number of people that have died in a population of a certain size. Incidence rates are defined exactly as mortality rates, where one just counts incident cases, that is, newly diagnosed cases of a particular disease. Meanwhile, the SMR is a measure of the mortality in a group of persons as compared to the general population. Finally, the survival after diagnosis of a disease is defined as the fraction of diagnosed individuals alive at a given time after diagnosis.