How to use the bayes factor calculator
Prepare the values for this calculation
Recover a Bayes factor from prior and posterior probabilities for the same hypothesis. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- Prior probability P(H₁): enter a value at least 0 and no more than 1. Use the unit named in the label or selected above. Do not leave this required field blank.
- Posterior probability P(H₁|data): enter a value at least 0 and no more than 1. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Bayes factor BF₁₀ first. With the unchanged example inputs, it is 4. Compare the additional metrics with the primary result; each has its own label and unit.
Compare and keep a useful result
For a controlled comparison, keep every other example input unchanged and change Prior probability P(H₁) from 0.2 to 0.3. The resulting Bayes factor BF₁₀ is 2.333333333333. This illustrates the equation’s response to that input, rather than a forecast of real events.
Read the chart’s axis labels and data table. A scenario curve changes one stated input while holding the others fixed; it does not add external observations. Use Download graph to save an available chart as a PNG.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Bayes Factor Calculator formula
BF₁₀=posterior odds/prior odds, where odds=p/(1−p).
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| Prior probability P(H₁) | minimum 0; maximum 1 | 0.2 |
| Posterior probability P(H₁|data) | minimum 0; maximum 1 | 0.5 |
Worked example
Using the example inputs above, the bayes factor bf₁₀ is 4.
- Bayes factor BF₁₀
- 4
- Prior odds
- 0.25
- Posterior odds
- 1
Results and assumptions
The output uses the bayes factor bf₁₀ convention and formula shown above. Match the entered units and source assumptions when comparing results.
Fix an input or unexpected result
Prior probability P(H₁) is required. Enter prior probability p(h₁) using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.