One yes/no question, one piece of evidence. This is the classic Bayes update.
Type percentages (20%) or decimals (0.2) — both work.
Several independent clues, applied one at a time — each answer becomes the
starting point for the next clue. (This covers both the CLI’s sequential and
naive modes; they agree on the final answer.)
“What if I took the same test again… and again?” Repeats one identical update as a sensitivity check — like asking for a second and third opinion.
Working from raw tallies — a study, a spreadsheet, your own records? Enter the counts and the percentages are derived for you.
Fill in the numbers on the left — or pick one of the worked examples below — and the updated probability appears here, live.
Try it on a real problem
Each card loads its numbers into the calculator above, so you can see the setup and then poke at it. Change one number and watch how much the answer moves.
The only three numbers you need
Every problem on this page boils down to the same three questions.
prior — the base rate
Before any evidence at all, how common is the thing you’re worried about?
“About 1% of people my age have this disease.”
likelihood — the hit rate
If the thing is true, how often does this clue show up?
“The test catches 90% of real cases.”
false-positive — the cry-wolf rate
If the thing is false, how often does the clue show up anyway?
“The test also flags 9% of perfectly healthy people.”