Bayesian probability explorer
See how a prior probability and a signal interact. A signal with false positives can still have a low posterior probability when the original event is rare.
Your inputs
Start with the example values, then replace them with your own. For tables, keep the vertical bar separators and use a dot for decimals.
A small or biased sample can give misleading results. This tool does not diagnose conditions or justify medical, financial or other consequential decisions.
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Result and workspace
Use the start button to open your result or activity.
How it works
P(event | signal) = prior × hit / [prior × hit + (1 − prior) × false signal].
Worked example
These sample results use the original example inputs in the form. They do not change when you edit your inputs; your own result appears in the workspace.
- Signal probability: 26 %
- Posterior probability: 30.7692 %
| Per 10,000 cases | Count |
|---|---|
| Event and signal | 800 |
| No event and signal | 1800 |
Read the result carefully
Results depend on the values and assumptions shown above. Change one input at a time to see its effect. A blank or unavailable result means the data does not support a calculation; it is not zero.