Before you start an experiment, you should have a very clear idea of who belongs in your experiment. A user question to answer before you start an experiment would be, "Considering the nature of the experiment, who should be put into this experiment?"
With that in mind, you'll be able to send the right experience/experiment, to the right user, at the right time.
How to think about your segmentation
One consideration is if you're very particular about who belongs in an experiment (as you should), if a user was assigned an experiment and then falls outside of the experiment, they no longer qualify.
A clear example for this is if, say, your experiment was to include only users who are:
- on iOS
- using appVersion = 1.0.0
- have made 0 purchases
If a user made a purchase (so they have 1 purchase), that user will no longer qualify for your experiment.
If a user upgrades to appVersion 1.0.1, that user will no longer qualify for your experiment.
If a user switches to the a new Android device and sells their iOS device, that user will never qualify for the experiment on the other device.
A simple way of thinking about your segmentation is that your filters is a net that you cast to include which users belong in your experiment.