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.