Typical scenario

A likelihood ratio was called donor probability

If a single-source DNA report describes its likelihood ratio as the probability that someone left the trace, the statistic's meaning changes. Competing hypotheses and prior odds may be omitted. Review the interpretation even if arithmetic is correct.

What needs to be clarified

Check whether the verbal conclusion reflects the LR or swaps probabilities of evidence for probabilities of source hypotheses.

Materials to prepare

  • Full conclusion and profile table
  • Calculation file with allele frequencies and model settings
  • Both hypotheses and population database description

What to compare

Reconstruct numerator and denominator and their database link. Check related-person alternatives and adjustments. Compare the wording with Bayes' formula: posterior odds require prior odds, which cannot silently be set equal or inferred from the DNA profile itself.

Possible outcome of the analysis

A review may supply conditional wording for evidential strength, locate the logical substitution and list assumptions needed to reproduce the calculation.

Limits of the conclusions

An LR is neither guilt probability nor unconditional identity proof. Review establishes no transfer mechanism, contact time or circumstances of deposition.