DNA evidence seems so damning, but could it be circumstantial?
Do cross-examining attys ever question lab reps to give the jury assurance on the quality of the evidence? A simple mix-up could easily link the wrong persons with the wrong crime scenes! Shouldn’t key Lab QA personnel be required to testify on their procedures to assure the jury all precautions were taken to safeguard evidence, data, etc? (Or, submit a written affidavit? …Can the jury see this?)
DNA evidence is like all evidence, it has its probative value in context. So if my blood is found on an axe which they say was used for the murder, its pretty damning, but if I point out that I lived on a farm, I used that particular all the time, its less probative, as it is also now possible that the blood got there through some other use.
Thanks, AK84, but I am asking about mix-ups in the DNA lab itself as opposed to how/where DNA was collected couched in a particular setting. Good point though…the DNA alone, like any one piece of a puzzle, does not tell the whole story.
I have not worked in a forensics lab, though I have worked in a pharmaceutical lab, an industry which is highly audited and run according to regulations laid out in criminal law.
There’s something called chain of custody, in which a piece of evidence is discovered, recorded, identified with a specific and unique ID, collected and transferred to storage/labs etc. The people who do this follow set procedures and basically put their name (signature and date) and, consequently, career and reputation, on everything that happens to that piece of evidence, so that it can be tracked right back to when and where the item was found. In pharmaceuticals, one of the guiding principles is that if you didn’t record it, it didn’t happen. You quite literally write down everything you do to a sample, and all work is linked back to the unique ID assigned to it.
Standard operating procedures and thorough documentation practices are - in theory - sufficient to prove that evidence was collected, handled and tested correctly.
Got you. As said above, you have certain set procedures. It is presumed in the court at the time of presentation that the procedures set are i) adequet and ii) have been followed. This presumption can be rebutted with sufficient evidence to the contarary.
This is the problem I have, it’s presumed.
Defense attorney, especially PDs don’t put much effort into disproving this.
Twice when I was working with a prisoners reform group, we had people who would make strong cases for error. Our group paid to have the DNA reexamined. In two cases the DNA was not a match. Yet two different DNA labs said, it was a 99.99999% certain match. Of course then the lab will admit to lab error.
I never figured this out. I mean DNA is on a piece of evidence. Lab says there is a near 100% certainty it matches. Then on retest, it no longer matches at all.
I could understand an inconclusive or something, and it’s not tampered with by the police because if so, then it’d still be there. If the cops planted some DNA the DNA would still be there for the retest. If DNA is so unique how does a lab reach that conclusion?
Unless those labs simply don’t do the work and pass it off.
In both cases the petition for retrial was made one denied and one reopened and reconvicted without use of DNA evidence.
But it shows me that mistakes happen and unless your defense attorney can prove absolutely that the chain was followed it’s suspect.
The procedures obviously include the proviso that you don’t work so a drop from A gets into B so as to erronously get a positive match; or mislabel the results pages so that you are comparing the readings from 2 A samples, not A and B. Either the labs were honestly incredibly sloppy, or they decided to say what the DA wanted to hear.
Another possibility is that a thrd sample contaminated both. Recall the CSI-NY episode (a little hokey) where a person working in the Q-tip factory without gloves was contaminating swabs, and they were getting positive matches all over the place. Maybe someone’s dandruff was showing up in all the samples, ha ha.
Basically, they put a tiny sample in an organic soup that will cause it to produce a huge volume of copies of the original DNA sample. (something recent - 20 years ago they needed a much bigger sample instead) So a tiny contamination could become the dominant sample DNA. Accidentally using the same tool in both samples could also cause erroneous results. Lab procedures of cleanliness and isloation are vital to quality results.
The trouble is that people take pretty much any DNA as conclusive proof of guilt.
Some procedures are known to be unreliable but still get used. DNA, like fingerprints, can’t be tested with complete accuracy.
And, of course, the actual evidence is the word of the forensicist testifying in court, little more than hear say. In fact at least a couple of forensic scientists have sent people to their deaths without ever really being questioned.
Joyce Gilchrist faked evidence to put people away, led to several innocent people being executed.
Fred Zane simply made it up because he was put in charge of a forensic lab without any idea what he was doing.
Not so hokey… I haven’t seen that CSI episode, but it’s clearly based on the Phantom of Heilbronn, a supposed serial killer whose DNA was being found connected to all kinds of cases, including the murder of a police officer. In the end, it was DNA contamination by someone working at the factory that made the cotton swabs used to collect the samples.
All the forensic scientist can say is whether two things match, and express the overwhelming improbability that the match is false. The science backing this up is very strong - it’s not merely a matter of opinion. If your fingerprints, blood, semen etc is found somewhere, it’s easy to prove. It is, however, not their job to say whether an individual did something illegal or not. The science only shows the source of material. Documentation, lab records, evidence of following established objective procedures only strengthen the validity of the results.
Sure, mistakes happen and some people falsify/mess with the evidence, but I’ll bet that they are not nearly as frequent as you imply. If a technician isn’t questioned, that is the fault of the defense, not the prosecution. It is the judge/jury that “sends people to their deaths”, not the scientists.
Unless the defence counsel has cogent evidence he can put forward that the tests results are not reliable, or the particular lab has a history of errors; it is going to be couter productive to challenge the evidence by claiming the test might have been contaminated or badly done. It will appear to the court that you really don’t have an answer and are acting in despertation; which will harm your case.
Even simply asking in XX, “the sample was contaimated” will result in a simple denial from the witness unless you can show therwise and in fact by doing so you have strengtened the prosecution case.
I am not sure what you mean by the first sentence - DNA evidence is always circumstantial. Circumstantial doesn’t mean weak or second rate. It just means indirect (where direct means evidence from someone who actually saw the crime happen). Circumstantial evidence may be weak or it may be devastating.
Generally, a mix-up in a DNA lab fails to safe. Let me explain what I mean. Let us suppose Smith raped a woman from whom intimate samples were taken and tested for DNA. Smith is suspected, investigated and found to have been in the area at the time, to look like the man the victim described and so on. Police take a DNA reference sample from him and get a match.
Now suppose there is a mixup in the lab. In this case the mixup takes the form that the reference sample from Brown was accidentally swapped with Smith. Now the reference sample that police believe to be Smith’s and is labelled Smith (but is in reality Brown’s) does not match. Smith is (on the face of things) exonerated, not implicated for this crime.
Instead, the intimate sample from the victim is registering a match from a guy who was apparently a suspect in (say) a robbery two states away (Smith’s DNA is mislabelled as Brown’s, remember).
And suddenly the same thing has happened in the case for which Brown was a suspect. It won’t take long for people to realise what has happened. And remember, police take many intimate swabs. It is almost always possible to go back to the original material from the intimate samples, retest the suspect from the beginning, and check that way.
Contamination hypotheses usually have the same problems. Semen looks different under a microscope than blood or saliva. So if a villainous policeman puts Smith’s reference sample into the victim’s intimate sample to implicate Smith, then the lab technician is going to be wondering why he is seeing blood and not semen under the microscope. If he does see both semen (from the real offender) and blood (from Smith’s reference sample), then the techniques used to analyse DNA have the effect of separating the blood and other cells’ DNA from the sperm DNA. (This is because the cell walls of sperm are much tougher than other cells like vaginal, blood or saliva cells, and so can be differentially ruptured to release their DNA.) No policeman is going to have a semen reference sample with which to contaminate the victim’s intimate sample.
In the case of blood at a crime scene (the OJ Simpson scenario) a miscreant police runs the risk that for no sensible reason two profiles will emerge (the real offender and the person whose DNA has been planted).
Once again, no-one says mislabelling, contamination and frank planting of evidence are impossible. But these people are not amateurs. In my experience they are very careful indeed to make sure the procedures in the lab make the risks they can control for very unlikely indeed. You can’t imagine the lengths they go to to prevent mixups and contaminations occurring, precisely because they are aware of the risks inherent in supersensitive testing.
I have fought cases where the defence have gone to utterly enormous lengths to assert that just such a thing must have happened - and they have failed. Not because jurors are stupidly credulous, but because labs have set ups that are designed to minimise error occurring and to detect it if it does occur.