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What is - Index Date?

Hello Everyone,

Can anyone please explain me what is index date from a layman’s perspective. I get totally confused. For ex, let’s say patient ‘A’ was observed from Mar 1st 2019 to Mar 31st 2019 for a drug that he takes everyday. In this case, the observation period is 31 days (1st-31st march). But what does the below term means?

  1. Index date
  2. Initial event date
  3. Restrict initial events

Apologies if my question is very basic, this is the first time I am dealing with healthcare data and quite confused about all these terms

Know 1. and 2, but don’t know 3 too :slight_smile:
Wish experts to give a precise explanation.

@qiongwang, Can you hep me understand term 1 and 2?


This video (up to the 40th minute) answers questions 1 and 2. Question 3 is answered around the one hour mark. However, I recommend watching the whole video as it also includes design principles, cohort definition examples and how to create these cohorts in ATLAS.

An older version of ATLAS is used in the video which means the terminology has changed. So, Restrict initial events in the latest version of ATLAS is Initial Event Inclusion Criteria in the old one .

Slides from the video above can be found here.


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