Database Match Probability
Definition
The adjusted probability that a coincidental match to a crime-scene profile exists somewhere in a database, accounting for the number of profiles searched. Calculated as approximately N times the single-comparison random match probability, where N is the database size.
- Field
- Forensic DNA statistics
- Formula
- Approximately N x random match probability
- N represents
- Number of profiles searched in the database
- Purpose
- Corrects for multiple-comparison search effect
Common questions
Why can't a single-comparison random match probability be quoted directly when a match came from a database search?+
A database search compares one crime-scene profile against many stored profiles simultaneously, so the chance of at least one coincidental match rises with the number of comparisons made, similar to the multiple-testing problem in statistics; using the single-comparison figure without adjustment understates the true probability of a chance match arising from the search itself.
Does a database search producing a hit mean the identification is weaker than a direct suspect comparison?+
The evidential weight is not necessarily weaker, but it must be calculated differently and reported using the database-adjusted probability rather than the single-comparison figure, and courts in several jurisdictions require this distinction to be made explicit in expert testimony to avoid overstating certainty.
Related terms
- Base Rate
- The prior probability of an event or proposition before specific evidence is considered. In forensic inference, the base rate is the probability...
- Bonferroni Correction
- A standard adjustment for multiple comparisons: the significance threshold for any individual test is divided by the total number of tests performed,...
- Ecological Fallacy
- The error of applying a statistical association observed at the population or group level to an individual, without verifying that the individual...
- Look-Elsewhere Effect
- The inflated apparent significance of a finding that arises when many features, tests, or subsets are examined and only the significant result...
- Multiple Comparisons Problem
- The inflation of the probability of at least one false-positive result that occurs when many statistical tests or pairwise comparisons are made....