Welcome to this new video. It's the part A of assessing the values of forensic results. As introduced by Tasha, Alex and I will discuss in this part, a very, very infamous case, the murder of the English exchange student, Meredith Kercher in Perugia, Italy. This is a complex case, and we cannot study it all here. We will present briefly the case and discuss one particular aspect, which is how to assign values of a DNA profile that is in a very small quantity. The analysis of small quantities of DNA often results in DNA mixtures and a profile composed of DNA contributions of more than one individual. DNA mixture needs special attention from all parties. As mentioned in the President's Council of Advisor on Science and Technology report on forensic science, the so-called PCAST report, it's crucial to note that the fundamental difference between DNA analysis of complex mixture samples and DNA analysis of single source and simple mixture lies not in the laboratory processing but in the interpretation of the resulting DNA profile. Method of interpretation need therefore to be fit for purpose. Now, before delving in into the specific of the Amanda Knox case, let's discuss briefly of the magic bullet evidence called DNA. Often for the general public and for the court as well, when DNA profile are found to correspond between a crime scene stain and the suspect in full or in part, then it's considered as tangible evidence towards is guilty, or at the very least, that DNA surely come from that person. Sure. A DNA match should be sufficient to prove that he's the source of the crime stain, right? No need to split hairs because a match is such an overwhelmingly powerful result, and DNA is unique, is it not? Well, no. In all seriousness, things are not as simple as they seem to be. We will shortly see together why. I see that you are being cheeky today, Alex. Unfortunately, as you are well aware, some personal things that are much represents the end of a story. That's not. What do we mean by match or by correspondence between the characteristic of the recovered DNA profile and that of our reference, the suspect of the victim, for example. Not being able to exclude a person as being the source of the DNA, that does not mean that this person is highly probable, the source of the recovered profile. If one follows the principle of interpretation described earlier and also in the MC guideline, one will know that there is a difference between the probability of the results and the probability of the propositions. We will have the opportunity to revisit this essential topic in the following weeks, but do keep in mind that the DNA correspondence does not mean that the person is the source of the DNA. Although, even if it's very rare to share the same DNA profile, one know that rare events happen from time to time. One knows, for example, that even if the probability of winning the lottery twice is very low, people still win twice the lottery. Yes, Franco. It is true that rare events do happen. But even so, most people think falsely, that if the probability of observing this profile if it came from some other person than say Jones, is only one in a billion, or one in a million. This is such an improbable result that the DNA must be that of Jones, and that it is splitting hairs to say that it is not. However, it is not splitting hairs. First, because as we said, rare events do happen, for example, some people get struck by lightning twice. Take for example the case in England of Mr. McDermott who won the lottery, five numbers plus the bonus ball twice. The odds against this happening would be five trillion to one, but it did happen. Five trillion to one against, that is a very small number. With DNA, because of possible dependencies between loci, the model that we use to assign probabilities is considered to be robust until numbers of one in a billion, say 10 to the power of 9. Here, the probability of winning the lottery twice was assigned as less than 10 to the power of 12. That is a thousand times smaller, and still we know that it happened. But what we have to remember is that the probability of the DNA coming from a person does not only depend on the probability of the result. It also depends on other elements of the case. We all heard stories of people who were in prison, while a DNA profile corresponding to theirs had been discovered on a crime scene. Luckily, for the person being in prison was their best alibi. We will see in the next weeks when considering the finger marks and DNA marks what one needs to say that the trace came from a person of interest. We will see what the necessary ingredients to identify a personal. It is a lot more than just the probability of the results, but let us leave this topic for now, and tell us, Franco, about the Amanda Knox case. Here are the facts of the case. Meredith Kercher was a British student on an exchange program who was murdered in Perugia, Italy on the night of November 1st, 2007 after having been sexually assaulted. One of them, roommate Amanda Knox and Amanda's boyfriend, Raffaele Sollecito, as well as another man, Rudy Goodday, was prosecuted for the crime, convicted, and sentences respectively to 26, 25, and 30 years in prison. After a series of appeals, Knox and Sollecito were acquired by the Italian Supreme Court in March 2015. The victim was found in a room in the late morning of November 2nd. The scientific police arrived on the scene on November 2nd in the afternoon and worked until November 5th. They came back on December 18th to complete their crime scene work. With respect to the result of forensic DNA profiling analysis in this case against Knox and Sollecito, discussion mainly gravitated around two items: a clasp of Kercher's bra on which the prosecution claimed to have found the victim's, and Sollecito's DNA and a knife found at Sollecito's apartment which the prosecutor claimed had the Knox DNA on the handle and the victim DNA on the blade. The chain of custody endow the items were handled and package were also the center of the debate. But let us concentrate on the aspect of our one shoulder says the values of DNA results. The expert declared that it was the DNA of Amanda Knox, Meredith Kercher, and Raffaele Sollecito. This is in disagreement with the basic principle of interpretation. Yes, sir. As we have seen numerous times, the evaluation also needs to robustly assess the probability of the results, that is the reported profiles given the two propositions of interest. However, when there is very little DNA, as it is the case here, one needs to be aware that the evaluation of the results given sub-source propositions, such as the DNA is from Meredith Kercher or from some unknown person, is more complex. Let us see these first with Tasha.
