The Best Uses for DNA: The Duke Case Continued

Another statement I disagree with from the original article:

The best use of DNA is excluding someone as the source of a particular sample," said Mark Rabil, a Winston-Salem lawyer who represented Darryl Hunt, a man freed in part by DNA evidence after serving 18 years in prison for a 1984 rape and murder.

I recoil at the use of the term "best". If DNA, as it currently existed, were "best" used as an exclusionary technique, then we could get by with a simple test that either said "match" or "no match". All of the statistics and studies of DNA profile rarity would be unimportant.

I won't belittle the benefit DNA has had for the wrongly convicted, but we shouldn't throw out the baby with the bathwater either. The primary benefit of forensic DNA technology as it exists today is not finding out who doesn't match the evidence, but understanding what it means when someone does match the evidence. The statistics that tell us that a profile is so rare it is found in only one out of ten trillion Caucasians allow us to interpret our results with confidence and present them honestly. We can then make intelligent statements about what fits best with the evidence.

If you read up on the current dust-up surrounding the admissibility of fingerprints into court under the Daubert standard, you'll understand why knowing what a match means is so important.

The Durham Lacrosse Team Rape Case

A rape allegation involving Duke University's Lacrosse Team has quickly become a high-profile case. Scores of articles can be uncovered by searching news.google.com for "duke lacrosse rape". I don't have an opinion as to what happened, as I was not there and I have not personally reviewed any of the evidence. However, some egregious misstatements in a recent article have attracted my interest.

One belongs to the "willing the evidence into existence" category:

Lawyers representing some of the 46 players tested said the tests found no matching DNA on or in the woman. They contend that the results prove that no rape or sexual intercourse took place.

But the prosecutor disagrees, and the case isn't settled.

According to a U.S. Department of Justice study, DNA evidence from an attacker is successfully recovered in less than a quarter of sexual assault cases.

A statement in the Boston Globe directly attributes this figure to the DA:

Nifong said prosecutors were awaiting a second set of DNA results, but did not say how those differed from the tests reported Monday. Nifong added that in 75 percent to 80 percent of sexual assaults, there is no DNA evidence to analyze.

I can't tell from the media reports if no semen and no DNA were recovered from the victim, or if semen and DNA were found and the DNA that was found did not match any of the Lacrosse team. The difference is critical, as one finding supports rape by someone else while excluding the Lacrosse players, while the other does not support rape by anyone. What I'm stuck with is this: if semen and/or DNA had been found at all, why make the excuse that it's rarely found in the first place?

I'm unable to find any study that supports the DA's contention that DNA is rarely recoverable from rape cases. I know of reports (see below) that state that DNA from rape cases is rarely submitted to crime labs, but none that DNA is rarely recoverable. These two statements are completely different. Historically DNA was simply not collected or even submitted to crime labs due to any number of reasons, including that the victim had no idea who the attacker was. In the days before convicted offender DNA databanks, there was simply no way to know who the attacker might be without an eyewitness identification of some kind. Suspectless rape cases were simply a dead end. DNA was available, but without a suspect for comparison the results appeared useless.

I suspect that the 25% statement is a misquoting of a National Institute of Justice Report entitled The Unrealized Potential of DNA Testing, which states that of all of the reported rapes, about 40% were investigated by police, 9% provided DNA evidence to crime labs, and in 6% of cases was the DNA actually analyzed. If we adjust the numbers to include only those investigated by police, about 22% had DNA submitted to crime labs, and around 16% actually had the DNA processed. I'll add that these numbers are nine years old, and pre-date most of the "Cold Hit" programs that analyzed DNA evidence from suspectless rape cases. I wonder what the figures would be as of 2006?

The critical statement is that these numbers do not concern how much DNA is recoverable but actually recovered and analyzed.

Or maybe the figure came from somewhere else, since the quoted figure is 25% and not 20%. Maybe they're misreading this National Institute of Justice document entitled Convicted by Juries, Exonerated by Science: Case Studies in the Use of DNA Evidence to Establish Innocence After Trial, that in part states:

"Forensic DNA typing laboratories -- as numerous commentators have noted -- encounter rates of exclusion of suspected attackers in close to 25 percent of cases."

Which covers how often the initial suspect is the wrong man, not how often DNA is recovered from a case.

I cannot find any reference to a study that DNA is rarely recoverable from rapes. If anyone knows of the study, please let me know by posting to the comments of this article. I'll post a follow-up if I can find the study.

For the sake of giving the devil its due, let's do some math.

Not finding DNA in a rape case tells us plenty even when we take the 25% figure at face value. Proclaiming that "DNA is only recovered in 25% of rape cases" argues that a negative DNA result is essentially meaningless. This only makes sense if we refuse to examine alternative reasons for recovering no DNA. As before, let's assume that frequency and probability are the same.

Examined formally, this would be:

Probability of no DNA when rape did NOT occur: 100% or 1

Probability of no DNA when rape did occur: 100%-25% = 75% or .75

Relative probability of encountering no DNA when rape did not occur vs. rape did occur:

1/0.75 or 1.3333333... or about 1.3333 to one.

Therefore, not finding DNA is more likely if no rape occurred. The relative difference is small, but real.

Another way to think about this problem is in terms of reasonable doubt. Would the fact that no DNA was recovered make you cautious enough to have reasonable doubts? Or more appropriately, if we were the accused party, would we want the 1.33333 to one odds to play in our favor?

Even if DNA is recovered as rarely as 25% of the time in rape cases, the "no DNA" result is more likely if the rape did not occur. Before anyone gets angry with me, remember that I am not saying it's more likely that the rape did not occur, I'm saying that the fact that we have no DNA is more likely if the rape did not occur.

The Core Question

From the previous post, the core question asked by the forensic analyst was:

How likely is the DNA profile we found if we assume the suspect committed the assault?

versus

How like is the DNA profile we found if we assume some unrelated person committed the assault?

How would we apply such thinking generally to non-forensic questions? Generalizing the core question looks like this:

How likely are the fact(s) I can truly verify if we assume one hypothesis?

versus

How likely are the fact(s) I can truly verify if I assume a competing hypothesis?

In the forensic case, the fact that can be verified and relied on is the resulting DNA profile. Everything else involved in the case the analyst was not around to witness and should be part of the "assumption" and not the facts.

In the general case, let's use a hypothetical model involving a contentious political issue. Let's say that the hypothetical state of Beatnikia bans all guns to alleviate their burgeoning gun crime problem. Some folks hate this law, claiming that it will disarm law-abiding citizens and cause crime to escalate. Other folks love this law, hoping that they'll finally be safe from gun-crazy criminals.

A federal study carefully monitors the results and finds that ten years after the ban was enacted that gun crime is roughly the same. What would be a clear-headed line of thinking? That gun crime would be lower if we hadn't disarmed the citizenry? That gun crime remained the same because gun nuts are hoarding their guns and selling them to criminals? Without further information, neither of these ideas are particularly clear-headed. These statements reek of confirmation bias, a beast discussed in an earlier posting.

the one fact we can verify is that gun crime rates are the same. Thinking forensically about that result might lead to the following questions:

How likely would that be if our law had any effect on gun crime vs. something else going on?

How likely would that be if one or the other group of concerned citizens were correct versus neither of them were correct?

How likely would that be if we really didn't understand the problem of gun crime?

This example sounds trite, but the core argument is profound. If we ask predictive questions only about the facts we can verify, we keep our heads even when things don't go our way. We no longer have to be "right" or "wrong", we merely transition from one honest idea to the next. Our theories, despite how dear they are to us, live and die on the facts we know are true.

It's a liberating idea. Imagine how much more interesting political debate would be if the participants first determined what facts they could rely on and weighed the probability of those facts given different sets of assumptions. It's a hopeless pipe dream to expect the talking heads who sell advertising for MSNBC and Fox News to embrace this idea, but most people aren't so completely compromised.

I'm not so naive as to expect some sea change in our political landscape. Politics will still be contentious and mean-spirited. Nonetheless, this "forensic" approach permitted thoughtful conversation and debate even when my fellow interlocutor disagreed with me completely. What's more, I almost always learned something from the discussion.

A Real World Example

Forensic thinking is best explained with a real world example. As unpleasant as they are, rape cases often involve straightforward forensic thinking. I apologize in advance for anyone who might be upset by my blunt and clinical discussion of an extremely painful crime. A male assailant in a typical sexual assault leaves semen either on or inside the body of the victim. The assailant's DNA can be extracted from the semen and a DNA profile determined. If the DNA profile matches a suspect, the analyst must now determine what this match really means.

The forensic analyst asks two questions:

How likely is the DNA profile we found if we assume the suspect committed the assault?

vs.

How likely is the DNA profile we found if we assume a random person committed the assault?

The first question is simple. If the person whose DNA profile matches the profile recovered from the victim committed the rape, we'd expect to find his DNA profile every time. Therefore, the probability of his profile being found at a rape he committed would be one.

To determine how likely the DNA profile would be if someone else committed the crime, we need to know how often we'd expect to find a person unrelated to the suspect with a DNA profile matching the crime scene profile.

Population studies allow us to estimate the rarity of a DNA profile. In this case the crime scene profile is quite rare, the probability of encountering it is 0.000000000001. How small of a probability is that? Imagine the odds of rolling a "6" fifteen times in a row on a standard six-sided die. Imagine flipping a coin and have it turn up heads 147 times in a row. Get the general idea?

With a solid estimate of how rare the crime scene profile is, we can now evaluate our two scenarios.

The probability of encountering the DNA profile if the suspect committed the rape: 1

The probability of encountering the DNA profile if some random person committed the rape: 0.000000000001

Computing a ratio of the probabilities: 1/0.000000000001 = 1,000,000,000,000 or roughly one trillion to one.

In this case, the DNA evidence we found is one trillion times more likely if we assume that the suspect committed the rape than if we assume that some unrelated person did the deed.

Notice how the alternate hypothesis uses the word "unrelated" person. What if the suspect has a sibling or other blood relative that may have also committed the crime? Then our calculation changes significantly, because the two scenarios become:

The probability of encountering the DNA profile if the suspect committed the rape

vs

The probability of encountering the DNA profile if a sibling of the suspect committed the rape

Thinking clearly about our assumptions is absolutely critical. For example, imagine if the suspect and victim were romantically involved. Finding a DNA profile matching the suspect would not be a surprise. The crime scene profile would not provide any useful information under such a scenario.

A Political Sidebar

My brother contributes to a political blog based in Cincinnati. One of the local newspaper publications published a rather curious editorial by Professor Norbert at the Center for Environmental Genetics and Department of Pediatrics & Molecular Developmental Biology at the University of Cincinnati. He had asked me to contribute a response, which is linked by clicking on the title above.

As is always the case, I feel like I have more to say on the subject, bloated windbag that I am.

The fundamental thrust of Professor Norbert's argument is that the natural world demonstrates that homosexuality is an aberration:

Genetically and evolutionarily speaking, homosexuality is not normal - no matter how Hollywood and other secular progressives wish to spin it. "Normality" (in genetic or evolutionary terms) is defined as reproductive success that leads to future healthy generations of the species.

Apparently this professor has never heard of Bonobo chimps. I doubt their homosexuality arose from watching "Designing Women" and a morally ambiguous attitude. Bonobo chimps are sadly in danger of dying out, but not due to their sexual practices. Apparently their meat is highly prized.

Liberals reading this statement may cry "Victory! Homosexuality is validated by the natural world!" Not so fast - if we use the natural world to find moral approval, we've just validated rape, murder, theft, deception, incest, and adultery.

Our dear professor wrote an embarrassing pile of rubbish. The natural world does not give a tinker's cuss for our morality. People may glibly pick and choose what parts of nature to select for moral reinforcement, but they are playing a loser's game.

As a postscript, Professor Norbert's confusion of "normal" with "natural" and "morally good" was dealt with in my original response.