Defence Proposition (Hd)
Definition
The alternative proposition, typically asserting that someone else is the source (e.g., 'the crime-scene DNA came from an unknown, unrelated person'). It occupies the denominator of the LR.
- Symbol
- Hd
- Position in LR
- Occupies the denominator
- Typical wording
- Crime-scene DNA came from an unknown, unrelated person
- Counterpart
- Prosecution proposition Hp occupies the numerator
- Field
- Bayesian evidence evaluation, likelihood ratio computation
Common questions
Why must the defence proposition be defined before the likelihood ratio can be computed at all?+
The likelihood ratio measures how much more probable the evidence is under one proposition than the other, so without a clearly stated Hd there is no denominator to compute, and the same evidence can yield very different likelihood ratios depending on what alternative proposition is being tested against.
Can there be more than one plausible defence proposition in the same case?+
Yes, and when that happens analysts may need to compute separate likelihood ratios against each plausible Hd, such as an unrelated stranger versus a close relative, because the two propositions lead to very different probabilities of observing the same DNA profile.
Related terms
- Kernel Density Estimate (KDE)
- A non-parametric smooth probability density function fitted to a set of measurements by placing a kernel (usually Gaussian) over each data point...
- 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...
- Log-LR (Log Likelihood Ratio)
- The natural or base-10 logarithm of the LR. Log-LRs are additive for independent evidence types, making them convenient for combining across disciplines....
- Prosecution Proposition (Hp)
- The proposition advanced by the prosecution, typically asserting that the defendant is the source of the questioned material (e.g., 'the crime-scene DNA...
- Random Match Probability (RMP)
- The probability that a randomly chosen unrelated person from the relevant population would match the evidence profile by chance. A very small...