Unity & In-App UA
Cohort
A group of users bucketed by their install day and followed over time, as opposed to an activity view that counts every event happening today regardless of when those users arrived.
A cohort is a group of users bucketed by the day they installed and then followed forward in time. The alternative, an activity view, counts every event that happened today regardless of when those users arrived. Activity tells you what the account did with your money yesterday; only the cohort view tells you whether a particular campaign, creative or placement is any good.
Confusing the two is the single most common source of wrong conclusions in in-app reporting, because a strong day of deposits from older cohorts can hide a weak week of new installs. Compare cohorts of equal age only, D3 against D3 and D7 against D7, and mark the as-of date on every export, since postbacks keep filling in behind you for up to about three days after the fact.
In buyer speech
“That spike is activity, not our new cohort, yesterday's deposits came from installs we bought a fortnight ago.”