The evaluation and presentation of DNA evidence
The final stage in any criminal case is the presentation of the evidence to the court. The way in which DNA evidence is presented has been, and still can be, a contentious subject. The evaluation process and the wording of the court reports and statements will be affected by both the judicial system and the prevailing approach to DNA evidence; this varies considerably between different countries. This chapter is designed as an introduction to the field and will guide the reader through the basics of the evaluation of DNA evidence.
Hierarchies of propositions
Any statement on the strength of the DNA evidence must be considered in the context of the case. DNA evidence should not be considered in isolation as it is affected by many factors like the type of biological material, method and time of deposition and the substrate on which it was deposited.
There are three hierarchies of propositions in relation to biological material that can be considered in a criminal trial [1,2]:
(1) Source level: from which individual did the biological material originate?
(2) Activity level: what activity led to the deposition of the biological material?
(3) Offence level: did the suspect commit the offence?
The hierarchies can be applied when, for example, considering a bloodstain found on the clothing of a suspect alleged to have committed an assault. The DNA evidence may assist in determining the most likely source of the stain. The size, shape and position of the stain may assist with determining the activity associated with the stain, i.e. how was it transferred and does this support an allegation of kicking or punching. The third hierarchical statement is that of offence: did the suspect commit the offence? The first hierarchical level can be addressed by DNA analysis and in many cases the second hierarchical level can be addressed to some extent by the forensic scientist, for example, by the interpretation of blood spatter patterns, but the third level is the provenance of the court and at no time should an expert witness comment whether the defendant is guilty of the offence. This is clearly the task of the court to consider [3, 4].
To answer the first question, there are currently three approaches to the evaluation of DNA evidence. The three approaches are termed:
the frequentist approach;
the likelihood approach; and
the Bayesian approach.
The starting point for each of these approaches is the frequency of the DNA profile (see Chapter 8).
The frequentist approach
The profile frequency is presented as a random match probability, which can be taken as the reciprocal of the profile frequency.
Before the advent of DNA profiling, the results of blood groups and protein poly- morphisms were expressed as random match probabilities, so for example, a re- port might state that ‘approximately 1 in 250 unrelated people will share this blood type’. It was natural to use this same wording with the advent of DNA analy- sis. In simple terms the frequentist approach describes the chance of a coinciden- tal random match. Random match probability (also called random occurrence ratio) is the probability of a person, selected at random, having the same profile as the defendant .
If we take a profile with a frequency of 0.000001 the random match probability will be:
The use of multiplex STR kits can lead to match probabilities of hundreds of billions— far larger numbers than there are people on the planet. In the UK the approach to such large numbers has been to employ a ceiling principle so that a figure of 1 in 1 billion is always quoted when describing a match based on a full AmpFISTR® SGM Plus® profile. There is little scientific merit to this approach but it is considered pragmatic. An example of a statement that is used in criminal reports in the UK is presented in the match probability statement.