ASU professor studies gun casing comparisons in trials
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Police and prosecutors use all sorts of methods to get convictions; some are more reliable than others. If someone’s on trial for the use of a gun in the commission of a crime, you might see prosecutors or defense attorneys use cartridge-case comparisons to prove innocence or guilt.
This involves matching the shell casings at a crime scene to the gun that ejected them. But this kind of test isn’t always reliable, and sometimes experts come back with inconclusive results.
In that case, it tends to point towards a negative match, but it isn’t always treated that way.
The Show spoke with Max Guyll, an associate professor in Arizona State University’s School of Interdisciplinary Forensics, about his research in this area.
Full interview
MAX GUYLL: What we really wanted to do was to assess the validity of cartridge case comparisons as a forensic technique for solving crimes. Since 2009, the National Academies of Science has had put out a report that drew a lot of forensic techniques into question as to whether they were valid or not, specifically saying that they lacked an empirical or scientific basis demonstrating their validity. And so one of our primary tasks in this study was to determine whether cartridge case comparison was in fact a valid, valid technique.
And it seems as though what you found is that maybe it's, it's not the best thing to be using.
GUYLL: I'd actually disagree with that statement. I don't know that, you know, to say the best means the best of all possible. So I don't know if that's true.
However, it is a very valid technique. When a firearm examiner makes a conclusive decision of either an identification or an elimination, saying that these two cartridge cases came from the same gun or that they came from two different guns, those types of decisions are almost always correct, more than 99% of the time.
So it is quite a, a strong and valid technique.
I guess, though, the problem is if, if it's not such a definitive match, right? Like if the results are inconclusive, it sounds like maybe those should be set aside and maybe not used is. Am I reading that correctly?
GUYLL: Well, certainly if an examiner says inconclusive, which can constitute a large number of decisions. In our case where we had 50% had come from different guns and 50% had come from the same gun. I'm sorry, about 20% of all decisions were inconclusive, and they were primarily the cartridge cases that had come from different guns.
So of course, when someone says inconclusive, you have to be very careful about what you try to make of that. And the thing that we found and that others have found as well is that when an examiner says inconclusive, it's not extraordinarily strong evidence, but it tends to point in the direction that the cartridge cases had been fired by different guns.
And that's, the evidence states that's associated with a factual innocence of a person.
So does that mean then that if a result is inconclusive, it sounds like that's more often than not, not the right gun.
GUYLL: That is what, what the data look like. Yes.
And so, of course, when you are taking a case to trial, an inconclusive result might not even come into the mix, because it's not compelling evidence one way or the other, might not be perceived that way.
What we think the real importance of our findings related to inconclusive decisions relates to, is how people think about inconclusive decisions, and specifically prosecutors. That if a prosecutor sends off some cartridge cases for examination and if they come back inconclusive, they should be aware that although it's not definitive, it does, does tend to point in the direction that that gun was not the one used in the crime. And that maybe they should rethink whether they have the right suspect or not.
Right. So in terms of the types of evidence that is collected from a crime scene and used to trials. How prevalent, how prominent are these cases. Like how, how often is it that, you know, these cartridge cases is, are, are used in a significant way to try to prove innocence or guilt, relative to all the other types of evidence that one might find?
GUYLL: I'm not aware of this specific rates. However, I do know that firearm analysis, cartridge cases, and, you know, bullet comparisons, that that is a very frequently used technique in the court system.
And you can imagine why, in America where handguns and firearms are so prevalent, and they're used in so often the commission of violent crimes.
So it's a frequently used technique, but that if it were inconclusive, probably an inconclusive would not be brought up at trial because it's not really clear to either the the prosecution or the defense that it's clearly helpful. And our point is that in many cases, these inconclusive decisions actually might be indicative that the gun is not the one that was used in the crime.
Right. Well, so is that kind of how you imagine this data being used by attorneys in these kinds of cases to, you know, instead of maybe trying to, you know, force through for lack of a better word or try to convince somebody of an inconclusive result being from the gun, they're saying it's from maybe take that off the table and, and try for other types of evidence or maybe don't try the case quite yet.
GUYLL: I think that when it comes back inconclusive, it needs to, it indicates that people have to take a real look at the case as a whole.
One thing is, you know, they have to consider both possibilities. That either they do have the right suspect and for some reason that the, the cartridge cases were just not clear and, you know, they can't tell us, they have to look to other evidence to that might indicate that the person is guilty.
On the other hand, they need to strongly weigh the possibility that they might have the wrong suspect or at least the wrong wrong gun, and then see if there might be other evidence that indicates, you know, another person might be the, the perpetrator.
So now that you have these data, what would you like to try to figure out next? Like what, what are the next questions that, that you would like answers to?
GUYLL: Well, the next thing we'd like to look at is to dive a little deeper into the inconclusive decisions to see under what conditions examiners are using them, why they're applying them for cartridge cases that happen to come from the same gun, and why they're applying them for cartridge cases that come from different guns, and trying to understand why it is that they might do that more so when the cartridge cases come from different guns.
We have, we know that they do tend to do that at least in validity studies, but we're not yet sure why. So we'd like to kind of get inside their thinking a little bit and to understand their decision making process a little better.
