Your core action is the thing people do in your product to deal with the problem they signed up to solve.

You want to start by shortlisting all the things people can do in your app to address their core motivation for signing up. 

To narrow it down, think about how often people experience the problem they’re dealing with. If the problem is filing your taxes, there’s not a lot you can track on a monthly basis that will tell you if people are successfully using your product to file their taxes this year. A yearly metric makes more sense here.

Airbnb use yearly metrics. People only go on holiday one or twice a year so it makes sense to track how many people book nights each year. 

On the other hand, a chat app like WhatsApp helps people coordinate on a daily basis. Tracking how many people send messages each day gives you a clearer picture of how useful it is.

A good core action will line up with how often the problem naturally occurs.

If you have data to run a correlation then the last step is to correlate your contending actions with long-term usage. The whole point of a core action is that when people do it they’re more likely to continue using your product in the long run.

If you still have options after you’ve run your correlations then always go with the metric that’s easiest to understand. A good metric that’s simple to grasp is better than something more accurate with lots of footnotes.