In order to understand the study o f epidemiology it is important to be familiar with several terms. Incidence refers to the number o f new cases o f a disease in a population during a specified time period. Age-specific incidence is defined by the number o f new cases in a particular age group divided by the total population in that age group. The proportion o f people who have a condition or disease at a specific time is prevalence. Relative survival is the ratio o f observed survival rate for a patient group compared to the survival rate expected for a population with similar demographics. Relative risk (RR) is the risk o f disease or death in a population exposed to the factor o f interest divided by the risk o f those who were not exposed. An increased risk o f disease is indicated by a relative risk greater than one, while a relative risk less than one shows a decreased risk from the exposure. Studies o f epidemiology need to be evaluated closely and the find ings weighed objectively since most are observational in nature and as such are subject to bias. Nonetheless, these studies are useful and can provide information regarding the underlying etiology o f disease states.