New metric in your Cohort Analysis: Cohort Size

You can now include the cohort size in your Cohort Analysis dashboards and reports. It’s a very handy KPI because it is the base of many important KPI calculations like sessions per user, retention rate, retained users, retention rate of paying users and so on.

As cohort size helps determine so many of your most important KPIs, it’s crucial to understand how this is calculated. As a reminder, cohort size is the user base for a specific period after an install or re-attribution; it is the number of users that can reach that nth day after install or re-attribution.

Suppose today is Monday, and that we want to look at the users who installed an app last week (ending with yesterday, on Sunday). And to make this a little easier, we’ll even name our users. Let’s say Alice installed our app on Tuesday, Bob on Friday and Charley on Sunday (the last day).

Remember, in a cohort analysis, we look at behaviour on a given day after install. The first thing we do is set each users’ install date as day zero. Any metric we compute after this is based on how many days have passed for each user after their install.

When we talk about cohort size, we mean the cohort size at a particular day-after-install. In our example, the size of our cohort at day 0 after install would be the full number of users: 3. The number remains at day 1 after install, but at day 2 after install we exclude Charley, since he only installed yesterday.

Thus, although his install day falls inside the time range for our cohort (last week), Charley still hasn’t had 2 days-after-install, and so he won’t be counted in the behaviour analysis for users who are 2 days-after-install. Similarly, Bob will be excluded from the analysis of users who are 4 days-after-install, because he installed only 3 days ago.

Note that since our first install for last week was on Tuesday, as of Monday this week no user could contribute to analysis of the 7 days-after-install behaviour for that cohort. This is the cohort size 7 days after install would read 0.

Check out our series on the metrics & measurement of cohort KPIs, Demystifying Cohort Analysis, for more.