Biology Reference
In-Depth Information
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 frequentist approach
The profile frequency is presented as a random match probability, which can be
taken as the reciprocal of the profile frequency.
1
profile frequency
Random match probability
=
(9.1)
Before the advent of DNA profiling, the results of blood groups and protein
polymorphisms were expressed as random match probabilities; so, for example, a
report 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 coincidental
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 [5].
If we take a profile with a frequency of 0.000 001 the random match probability
will be:
1
000001
=
1 in 1 000 000
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.
If we take the profile from Chapter 8, after correction for subpopulation effects
Random match probability
=
0
.
(
θ
=
0.01) the random match probability will be:
1
Random match probability
=
=
1 in 4 098 360 656 000
0
.
000000000000244
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 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.
