Cohort

One locked combination of variants, one per factor, that a visitor can actually be assigned to.

A cohort is one locked combination of variants, exactly one per factor, for a campaign. If a campaign has two factors with two variants each, it has four possible cohorts: one for every combination. A visitor is never assigned directly to a factor or variant; they're assigned to a cohort, which is the whole bundle at once.

Cohorts for a full-factorial design are generated automatically as the cross-product of every factor's variants. One cohort in every campaign can additionally be marked as the control cohort: the baseline every other cohort's lift gets measured against.

Exactly one cohort per campaign can hold that control designation at a time, enforced at the database level rather than left to convention; two cohorts each claiming to be the baseline is exactly the kind of ambiguity that makes a reported lift number impossible to trust.