dear all，
when I use the atlas to construct a cohort, I found I cannot understand the purpose of the cohort exit, Just like when I read the book of OHDSI, I am also confused about the sentence ‘Cohort exit strategy will impact whether a person can belong to the cohort multiple times during different time intervals’. So, what is the mean of the sentence?
To my understanding, a cohort consists of persons and their respective events. It just happens that a lot of cohort definitions will be designed to have one event per person. But you can design a cohort that can have one or more events per person so a person can exist in the cohort multiple times. Also, have in mind that these events do not overlap. That’s why the sentence says during different time intervals. You can read more about the cohort exit strategies available in ATLAS in chapter 10.7.5 Cohort Exit Criteria from the book of OHDSI.
By default, when a person enters the cohort, the person remains in the cohort until the end of observation. A person can not enter a cohort again, unless they leave the cohort. So, if you do not specify that a person leaves the cohort after the entry event (say, after 30 days), then the person will not have a second cohort episode within the cohort.
Think of it like this: You are a person, and there is a room you can enter, but you can enter that room if you have the correct credentials. Once you enter the room, you can’t ‘enter it again’ without leave the room first. So, are those credentials a ‘life time pass’ or only good for 30 days? if it is a life time pass, then you never leave the room. If it is a 30d pass, then you get kicked out after 30 days unless you re-certify for another 30 days.
Make sense? The person is the patient, and the room is the cohort. The ‘credentials’ are all the inclusion criteria you must qualify for in order to be admitted. The ‘Cohort Exit Strategy’ will define the mechanism that will get you kicked out of the room: one mode is ‘after a fixed number of days from the entry event’. The default mode is ‘you never get kicked out’. If you never get kicked out, then you can never have a second ‘entry’ into the cohort.
-Chris
Just one more thing, I wanted to answer the question from the Topic’s title:
What is mean of the cohort exits? If the cohor exits will influence the num of person in cohort?
Cohort exit criteria doesn’t impact the number of people in the cohort. If the person gets into the cohort, they are in the cohort. Cohort exit will not reverse that. The point of the statement:
Cohort exit strategy will impact whether a person can belong to the cohort multiple times during different time intervals
This is to remind the researcher that cohort exit strategy will determine if a person can appear multiple times in the same cohort at different time intervals.
Thank you very much，Chris, Your every explains about my questions are the easiest to grasp and master for me!