suspect analysis with respect to the clasp
To all,
First, my most recent message on the DNA on the bra clasp is
here. A proper analysis of the bra clasp would include an identification of the stutter peaks, which are mentioned in the message linked above. It also contains some references to the problem of subjectivity in the interpretation of DNA mixtures. Second,
here is a good discussion of how to minimize observer effects in DNA forensics. Note that the proposal indicates that the investigator should not have access to the reference samples. In other words, one wrong way to analyze an evidence sample would be to lay reference samples on top of it and look for matches. As far as I am aware, it would be OK to do so after the analysis were complete, simply to present the data.
Third, here are some thoughts on the “Professor Tagliabracci then maintained that this suspect-centric method was detectible in Dr. Stefanoni’s report and presentation because, he affirmed, it was a case of ‚forcing the profile obtained < eliminating or leaving out alleles [257] solely for the purpose of making that profile compatible with Raffaele Sollecito’s profile‛.” (Massei Report, p. 241)
As I interpret his remarks, Dr. Tagliabracci criticized Dr. Stefanoni for choosing smaller peaks in the clasp profile that happened to be in Raffaele's reference profile over larger peaks that did not. If Dr. Stefanoni did so because she had prior knowledge of Raffaele’s profile, then that analysis is open to question, as indicated above. Dr. Tagliabracci implied that this was a problem. “He pointed out that that there is a significant subjective element in reading the electropherograms. He focused in particular on locus D5S818, in which two principal alleles are present; together with a third peak with a height of 108 RFU; as this is higher than 50 RFU, it should have been considered an allele. Forensics [la Polizia Scientifica] did not, however, consider this to be the case; instead, they considered the 65 RFU peak to be an allele and observed that, in this way, a compatibility with Raffaele Sollecito’s profile resulted, which otherwise would not have been the case (page 59). With reference to this, Professor Tagliabracci repeated that there was a forced interpretation, which was typical of a suspect-centric attitude (page 60).” (Massei Report, p. 242) The quotation above only gives the peak height in RFU, not the number of repeats, which gives the location along the time (x) axis. I do not know which peaks are meant.
The Massei report gives several instances where Dr. Tagliabracci differed with Dr. Stefanoni with respect to the interpretation of certain alleles. However, it is possible for DNA belonging to Raffaele to have highly unequal peak heights, namely that the non-Meredith DNA is in the low template number range. This could lead to large disparities in peak heights in the two alleles within any locus because of stochastic effects. If the low-template number explanation is invoked, then the standard protocol for dealing with low template DNA is to run the sample at least twice. Dr. Tagliabracci noted this issue. (Massei report, p. 240)
My best guess on the clasp is that there is likely to be a partial profile corresponding to Raffaele’s DNA (but possibly not a full profile), and there at least one other contributor as well, apart from Meredith obviously). I think that Dr. Tagliabracci’s critique of the analysis is probably correct, but the more important question is how did the DNA get there. I think that secondary transfer, contamination, and evidence-tampering are all roughly equally likely.
I am suspending my commenting in this thread indefinitely. I can be reached via PMs for the time being.
