We want to reserve a control group of users in the welcome emails experiment who will not receive emails. This will allow us to see whether sending the emails has an impact on key newcomer metrics, like activation, retention, and productivity.
To do this, we should refrain from exporting a random 20% of the users that we would otherwise export. They will be users who have email addresses and opted-in (all the criteria listed in T303780), and therefore will be statistically the same as those who do get emails. In order to do the analysis, we'll need to know exactly which users are in this control group. An open question for the engineers is how to keep track of that.