Evaluating and Presenting DNA evidence

• Evaluating and presenting DNA evidence
• Final stage in a criminal case
• Match probabilities
• Should be evaluated and reported correctly so that they do not mislead the jury.
• DNA evidence only assists in determining the most likely source of biological evidence
• Likelihood ratios
• The Bayesian Approach
• The Frequentist Approach
• The profile frequency is presented as a random match probability, which can be taken as the reciprocal  of the profile frequency
• When using the several loci, the product rule gives match probabilities of hundreds of billions.
• In the UK, the ceiling principle is applied - 1 in a billion
• expresses the probability that a person, selected at random, has the same profile as the evidence.
• Straightforward
• Easily understood
• Does not consider two propositions
• Not very scientific
• Random Match Probability = 1/profile frequency
• The two fallacies
• Defendants Fallacy
• Assumes that every person with a matching profile has equal chances of being guilty.
• Prosecutors Fallacy
• Assumes that there a low probability of false matches does not mean a low probability of some false matches being found
• Two errors can be made if the wording is wrong