I have been ruminating on this passage for a little while, and I am still trying to understand much of it. What does thoughtful mean by "random people"? The instances of contamination with which I am familiar are usually cases where another person's DNA gets transferred onto an item of evidence or comes into the process downstream of sampling. The person could be a lab technician, suspect, or alleged victim in another crime, but there is enough of a profile (whether complete or not) to obtain an identification of that person. The notion that contamination only leaves partial profiles is at best dubious, as I have discussed previously. Moreover, such a blanket statement runs counter to an axion of DNA profiling, that the mere presence of DNA does not indicate the time or manner of its deposition.
It seems to me that thoughtful may be following the lead of Dr. David Balding, who is a mathematician whose area of expertise is the calculation of probabilities in DNA profiling, not the collection of forensic evidence. The BBC quoted Professor David Balding: '"Every crime sample that was ever collected was contaminated. Even in the most pristine conditions in a laboratory, you cannot have a DNA-free environment,' he says.
"The point is you have to allow for that to do a correct evaluation of the evidence; all of that kind of contamination just isn't a problem, as it's not going to match. The only contamination that matters is something that would have got the suspect's DNA.'"
This passage presents a number of problems. One is that if a lab technician deposits the DNA from himself/herself or a suspect or victim (whose DNA happens to be in the lab) onto an item, a juror is entitled to discount the quality of work from that lab. That sort of contamination certainly should be a problem for the prosecution. Two is that Dr. Balding does not explain what happens when the FP do not collect reference profiles from other people at a home where the crime took place or when they do not keep a staff elimination file (DNA reference profiles from staff). Dr. Balding does not sufficiently explain what he means by match. Match whom and under what circumstances? It would be difficult to interpret the finding of unknown profiles on an item of evidence if the FP's collection of reference profiles were deficient in this way, as is true in this case. Three Dr. Balding does not explain how a person became a suspect in his scenario. Was it from the DNA evidence itself? If the DNA evidence is faulty or weak for any reason, then the presence of a person's DNA on an item is less meaningful or not meaningful at all.
Let's apply Dr. Balding's criterion to the Duke
lacrosse non-rape case. David Evans' DNA was on some plastic fingernails found in the trashcan in the bathroom of his home (where the ill-judged party took place). The DNA of two other men is also evident in the YSTR profile, as discussed by Paul Giannelli. No one has ever publicly identified the other DNA donors. Since Evans was a suspect and since the profiles of the other party attendees were known, we can tentatively conclude that the other profiles came from unknown individuals. According to Dr. Balding these profiles mean nothing. However, such a position fails to answer the question, "If the other profiles came to be on the fingernails in a way that was unrelated to sexual assault, then how do we know that the same is not true of Evans' DNA?"
Let's also apply his reasoning to the Jane
Mixer case. John Ruelas was only four years old and living in a nearby city when Ms. Mixer was murdered. Therefore his DNA on her body cannot be evidence of his culpability in her murder. Therefore, we can ignore it, right? I don't think Gary Leiterman would agree. His DNA was found on Mixer's clothing, and he was convicted of her murder, largely (perhaps entirely) on the strength of the DNA evidence. What Dr. Balding has done is to allow prosecutors to ignore inconvenient DNA.