Define period prevalence. Contrast period prevalence with point prevalence. What is the distinction between period prevalence and incidence during the same period?

What will be an ideal response?

A period prevalence is the number of existing cases during a specified time period.  The time period is often a year in length but can be any time period.  The criteria for an existing case during the time period are that if someone has the disease at any time during the time period, they would be counted.  The ascertainment of cases during a period prevalence should be an ongoing process or at least have the ability to determine whether anyone in the observed population had the disease during the time period.  So unlike point prevalence, a disease should be counted as a case if the person had the disease during the time period even if they don't have it at the moment the counting takes place.  Also, period prevalence can be confused with incidence during the same period since a person may not get the disease until the end of the time period.  The distinction between period prevalence and incidence during the same period is that to truly be counted as incidence, the investigator must have a method to determine that everyone in the population is actually disease free at the beginning of the time period.  If there is not a process to ensure that the population is disease free at the beginning of the period, then those with disease during the time period must be counted as period prevalence since it can't be certain that they are new cases.