There are multiple definitions - click on the translation that matches your context.

a posteriori

latinsk uttrykk

full

a posteriori

adverb

Definitions of a posteriori

English

done in a way that is based on reasoning from known facts rather than assumptions about what might be true; with hindsight (=the opportunity to understand or judge something that has happened using knowledge that you have gained since it happened)

Therefore, the study populations may differ in some characteristics that cannot be determined a posteriori.