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Hierarchy of propositions

Dr Gavin Turbett, hierarchy of propositions, offence level proposition, forensic scientist.
The last key point I’d like to talk about is known as the Hierarchy of Propositions, and this concept originated from the United Kingdom in 1998, and the idea was that there is a framework that is used to assess scientific evidence and that that framework must be clear, in that it can be explained to the court and is understandable, that it is balanced and it must be seen to be fair and unbiased, and that it is logical and that the options that are used to assess the evidence make sense and are reasonable given the information relating to the case and might form reasonable hypotheses that would be formed by both the prosecution and defense in that particular case.
So there are three primary levels and we’ll go through these one at a time. Level one is the source of the DNA, and the question we’re asking here is who is the likely donor of the biological material, whether it’s blood or semen or skin cells? And these propositions, or this proposition is based entirely upon the laboratory results, and it would come from the results that the laboratory has generated from testing the crime scene material and the reference samples. So, the forensic scientist from having done their testing should have enough information available to them to be able to provide information at this level.
They’ll know information about how common each allele is at each particular locus in the relevant populations, and they can use that information to assess the DNA evidence.
The next level is activity, and the question being asked here is what actual thing happened or what activity occurred that allowed the biological material to be present at that location from wherever it was recovered from. So, for example, if there’s blood on the shoe of an accused person, did they get that blood on their shoe because they kicked the victim? Now, a forensic scientist might be able to assist and might be able to provide information here under certain circumstances, but they will typically need a lot of additional information before they can make any comment at about a proposition pitched at this level. Any assumptions that the scientist used would have to be clearly stated.
It’s quite possible that they would have to rely on either significant prior experience and/or published research. They may even find it necessary for any particular case to perform some additional simulation studies and do their own research relevant to that particular case before they have enough information to be able to make a comment about a particular activity level proposition. And finally, we’re at the top level, level three, which is the offense, and the question here, of course, is what offense actually took place? Was the offense committed by the defendant or someone else? This is what the court wants to know, and of course, what the court must ultimately make a decision about.
The forensic scientist must never speak to propositions at this level, either verbally, on the stand or in any court report that they issue. Because ultimately, the court has to make that decision of whether the individual is guilty or not guilty and that the role of the court, the decision maker, mustn’t be supplanted by the forensic scientist. The hierarchy is important because people are not convicted on the basis of a DNA profile, they are convicted on the basis of the activity that caused that DNA profile or that biological material to be present. It’s the basis of the activity that the DNA profile actually represents. So, the court might want the scientist to say more. Is it blood?
Who is the blood from? How did the blood get there? When did the blood get there? And did he leave the blood when he committed the offense? So if you think about each of those questions-is it blood? That’s a level one source proposition, and yes, the scientist, that is their role and they should be able to provide the court with information to confirm or refute as to whether or not it’s blood. Who is the blood from? Again, the scientist should be able to assist with that. But how did the blood get there? That is becoming much more complicated because we’re now talking about an activity level proposition. And it may not be possible for the scientists to assist.
There may not be enough information available for the scientists to make comment there. When did the blood get there? That is very difficult for the forensic scientist to answer, and it may not be possible at all for them to make comment there. Simply testing DNA and getting a DNA profile doesn’t allow you to tell when that bloodstain was actually deposited. And then finally, did he leave the blood when he committed the offense? Well, that is a level three offense level proposition and again, is completely outside the domain of the scientist.

What are the different levels of information that a forensic scientist can provide from the examination of biological evidence? Can a forensic scientist comment on the guilt of an accused? What does science allow a forensic scientist to opine on? It is essential that judges and lawyers know the propositions on which a forensic expert can comment on, so that they can scrutinise if the expert has overstated their claims in a report. Let us explore these issues with Dr Turbett in this video.

In your experience, do you know instances where a forensic scientist has opined on areas that were beyond their expertise? How did the court respond to such an expert and their opinion?

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Decoding Forensics for Legal Professionals

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