Study Shows Blood Pattern Analysis Is Just More Guesswork Posing As Scientific Evidence
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Another form of evidence used in criminal cases is being called into question. The latest (via CJ Ciaramella) to receive the dubious honor of being designated "dubious" is blood spatter analysis. This brings it in line with a long list of other things long-considered (and, in too many cases, still considered) to be evidence worthy of introducing into a court of law, joining bite mark analysis, hair analysis, um… pair of blue jeans analysis... and even the old standby, DNA analysis.The problem with all of these sciences is that they're mostly subjective. Sure, they look pretty science-y. A lot of math and charts and lab coats and computers are scattered all over the place. Inscrutable printouts are carried by expert witnesses with years of experience under their belts. They show up in court and make claims about certainty of matches or probability of X contributing to Y, much of which can't be easily contested because, as mentioned earlier, the results are open to interpretation.Despite this, a lot of what's called forensic science still ends up being used as evidence in criminal cases, even though it's more accurate to refer to it as forensic guesswork. Blood spatter analysis is no exception. This study [PDF] for Forensic Science International says the lack of solid standards in the blood pattern analysis field have resulted in experts looking at the same blood patterns but all seeing something different.
We investigated conclusions made by 75 practicing bloodstain pattern analysts on 192 bloodstain patterns selected to be broadly representative of operational casework, resulting in 33,005 responses to prompts and 1760 short text responses. Our results show that conclusions were often erroneous and often contradicted other analysts. On samples with known causes, 11.2% of responses were erroneous. The results show limited reproducibility of conclusions: 7.8% of responses contradicted other analysts.That's a problem. How big of a problem is unknown. Actually, a lot about this is unknown. Differing opinions on blood spatter evidence can turn a suicide into a homicide or turn a killing from an act of self-defense to a murder. There's actual freedom riding on these interpretations, so it's crucial they be correct. And yet no one in the field (or in the court system) seems too concerned about ensuring this evidence is correctly analyzed.
Although BPA has been admissible as expert testimony for more than 150 years, the accuracy and reproducibility of conclusions by BPA analysts have never been rigorously assessed in a large-scale study.In 2009, the National Research Council condemned blood pattern analysis as "more subjective than scientific." Nothing changed. In 2016, it called for testing of error rates in criminal forensic science. Again, nothing changed. Small studies were performed but nothing on the scale of this one, which involved 75 practicing blood pattern analysts and 192 samples. The results are far from encouraging. In fact, they point to enough of a margin of error that this sort of analysis should be prohibited from being introduced as evidence until standards are developed and put into practice that eliminates the subjectivity on display here.But, given the 150 years of use and the minimum of interest in developing standards, this will also likely lead to nothing changing. What this research does do, at least, is give defendants something to use to challenge questionable evidence and questionable statements by prosecutors' expert witnesses.
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