Evaluating and Presenting DNA evidence

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  • 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.
          • Advantages
            • Straightforward
            • Easily understood
          • Disadvantages
            • 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

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