Well … you knew that sooner or later we’d get around to “blood spatter”. This is also sometimes called “blood pattern analysis” (BPA). As the name implies, BPA is the analysis of patterns made by blood that has been expelled from the body by a violent act – stabbing, shooting, beating. These patterns can be used to help reconstruct the violent event, and can provide information about type of injury sustained, movement of a victim, angle of a shooting, location of attack, etc. Unfortunately, BPA is one of the forensic disciplines least based upon real science. It has evolved from a collection of anecdotal and empirical observations that have resulted in potentially flawed inductive inferences about the fluid dynamic behavior of blood outside the body.
Now, like all forensics, BPA is not useless. There is certainly legitimate information that can be garnered from an analysis of blood patterns at a crime scene. But, again, as with all forensics, it must be used within the bounds of what is scientifically supportable.
I’ve heard BPA experts speak, and they try to dress it up as science; even citing “experiments” that they say support their theories. Without going into a lecture about the ‘scientific method’ and ‘design of experiments’, I can say that none of the “experiments” I’ve seen have been properly controlled scientific experiments conducted with a single independent variable and rigorous hypothesis testing. For example, anyone familiar with fluid dynamics can tell you that one of the major variables determining the behavior of a fluid is viscosity. Did you know that the viscosity of blood can vary by a factor of 2-to-1 depending on the hematocrit level (volume concentration of red blood cells)? I am not aware of any BPA experiments ever done that have controlled for hematocrit level.
The results of the simple experiments that have been done are taken by BPA “experts” and extrapolated way beyond what is scientifically supportable – and this results in wrongful convictions.
A classic example of BPA gone wrong is the case of Warren Horinek in Ft.Worth, TX in 1995. It’s rare that BPA is used as the sole evidence for conviction, but here’s a case where that happend. Below is a link to a Texas Observer article detailing the case. As you will read, it’s a bizarre case in which even the DA and the investigating officers testified for the defense, but blood spatter testimony from an independent “expert” resulted in a conviction – and it was wrong.
The closing paragraph of the Texas Observer article is chilling. “… many wrongful convictions — whether due to faulty blood-spatter analysis, misread fingerprint matches, shoddy lab analysis, or junk arson science — disappear into the shuffle of the criminal justice system. No one knows about them. And there’s little chance they’ll ever be overturned.”
And here is a link to another brief example of the same kind of BPA error:
The NAS Report was particularly critical of BPA. (I hope by now you’re familiar with the “NAS Report”, but if not, you might want to check out my previous post on the subject: http://wrongfulconvictionsblog.org/2012/02/20/the-sorry-state-of-forensics-in-the-us-and-perhaps-the-world-2/)
The NAS Report Ch.5 presents a summary evaluation of many of the forensic disciplines and BPA is covered on pages 177-179. Here is just an excerpt from that section: “In general, the opinions of bloodstain pattern analysis are more subjective than scientific. In addition, many bloodstain pattern analysis cases are prosecution driven or defense driven, with targeted requests that can lead to context bias.” And, “The uncertainties of bloodstain pattern analysis are enormous.“