Biology Reference
In-Depth Information
transposed the condition of the hypothesis and it was one of the reasons that the
conviction was quashed by the Court of Appeal.
As another example, consider the following dialogue from
R
v
Doheny and Adams
(
R
v
Doheny and Adams
[1996] EWCA Crim 728):
Q. Is it possible that the semen could have come from a different person from
the person who provided the blood samples?
A. It is possible but it is so unlikely as to really not be credible. I can calculate;
I can estimate the chances of this semen having come from a man other than
the provider of the blood sample. I can work out the chances as being less than
1 in 27 million.
Instead of estimating the probability of semen (crime scene sample) matching
blood (of the suspect) if the suspect was innocent, the DNA analyst was estimating
the probability of the semen matching the suspect's blood if the suspect left the
semen stain. For the population of the UK a match probability of 1 in 27 million
means that other persons could have a matching profile and without other supporting
evidence may not in itself provide sufficient evidence against the defendant. This
however is a matter for the court to address and not the scientist. The prosecution
expert witness thus inadvertently enhanced and misrepresented the probative value
of the evidence [9]. It is therefore imperative that in order to avoid the prosecutor's
fallacy, the scientists write their report carefully, and while answering any questions
in the court keep their match probability or likelihood statements conditioned on 'if
the defendant was innocent'.
Defendant's fallacy
If the match probability for a DNA profile was 1 in 27 million as it was in the case
against Doheny, the defence could argue, for example, that three people in the UK
might match the crime scene profile, the probability of the defendant being the donor
of the crime scene sample is therefore only one-third, which is insufficient for proof
beyond reasonable doubt. The issue with this statement is that nothing is known of
these three people; whether they exist, where they live, what age they are, what sex
they are and what opportunity they might have to leave their DNA at the scene.
Comparison of three approaches
The high statistical values that are attached to DNA profiles might seem intimidating
and can unduly enhance the probative value of DNA evidence. This had led to heated
debate over the way in which the evidence should be presented to a court.
