Table 1.

Types of prevalence and their definitions, from Porta (2014).

Type (sub-type)	Definition
Point prevalence	The proportion of individuals with a disease or an attribute at a specified point in time
Period prevalence	The proportion of individuals with a disease or an attribute at a specified period of time. To calculate a period prevalence, the most appropriate denominator for the period must be found
One-year prevalence	The proportion of individuals with the disease or condition at any time during a calendar year. It includes cases arising before and during the year
Annual prevalence	The proportion of individuals with the disease or attribute at any time during a year. It includes cases of the disease arising before but extending into or through the year as well as those having their inception during the year. Only occasionally used
Lifetime prevalence	The proportion of individuals who have had the disease or condition for at least part of their lives at any time during their lifecourse