But you can consult another all-encompassing database and compare the lists, and modify the original database accordingly To the original question: all it says is if an unknown quantity can be determined. You already proved as much by stating your database was incomplete (you knew that you did not know). Inasmuch as there is an awareness of not knowing (you knew your database did not include non-customers), there is an opportunity to correct this (i.e.- make the unknown quantity known). Knowing that you do not know is knowledge in itself. Too much recursive reasoning, now my head hurts.