Bayesian Reasoning
Definition
A framework for updating beliefs in the light of evidence. Prior probabilities are combined with the likelihood of the evidence to produce posterior probabilities. Forensic scientists use Bayesian logic to quantify how much evidence changes the probability of a conclusion.
- Type
- Belief-updating framework
- Combines
- Prior probability and likelihood of evidence
- Output
- Posterior probability
- Forensic use
- Reconstruction and probabilistic case analysis
Common questions
How does Bayesian reasoning apply to crime scene reconstruction specifically?+
A reconstruction often has to weigh competing scenarios, such as an accidental versus a staged scene, and Bayesian reasoning gives a structured way to update how probable each scenario is as each new piece of physical evidence, like bloodstain pattern or trajectory data, is added.
What is a common misuse of Bayesian reasoning in forensic contexts?+
Treating the posterior probability of one piece of forensic evidence as if it were the probability of overall guilt, when in fact it should combine with the prior probability from the rest of the case, is a frequent misstep; the forensic scientist's likelihood ratio and the fact-finder's overall probability of guilt are distinct quantities that should not be conflated.
Related terms
- Activity-Level Interpretation
- An opinion about what activity produced or deposited the evidence, rather than simply what the source of the evidence is. Activity-level conclusions...
- Hierarchy of Propositions
- A framework that classifies forensic questions at source level (where did this come from?), activity level (what was the person doing?), and...
- Hypothesis
- A proposed explanation for observed evidence. In reconstruction, competing hypotheses are evaluated against the physical facts and background knowledge. A good hypothesis...
- Likelihood Ratio (LR)
- The ratio of two conditional probabilities: the probability of the observed evidence given the prosecution's hypothesis (same source), divided by the probability...
- Reconstruction
- The stage at which all analytical findings are assembled into a coherent account of how an event unfolded. Reconstruction draws on scene...