Study Shows Blood Pattern Analysis Is Just More Guesswork Posing As Scientific Evidence

from the blood-suckers dept

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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Comments on “Study Shows Blood Pattern Analysis Is Just More Guesswork Posing As Scientific Evidence”

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Anonymous Coward says:

Re: How big of a problem?

And what is the sigma there?

The real problem is there are no standards, and there are way more than 75 BPA experts. The error rate from the study is both new and unacknowledged, and people will look at 10% like a 10% chance of rain and think "oh well certainly with 10% chance, there isn’t an error" while still playing the lottery or going to Vegas. People are bad at this.

And let’s never mind the analyses which say whatever the cops and prosecutors hinted that they wanted them to say. That doesn’t show up in a statistical test.

BPA Guy says:

I work in this field and you’re not wrong. The field can’t even agree on how to classify or name things. Can’t do much science like that…

It would be a lot more useful to look at various parts of the work (for any field in forensics actually) and say "this analysis is subjective" and "this analysis is objective". Both have their place, but they should be more clearly delineated – especially when presenting evidence.

And IMHO, most of the work in BPA is subjective.

This comment has been deemed insightful by the community.
Pixelation says:

Re: Re:

"And IMHO, most of the work in BPA is subjective."

This is the rub that prosecutors hate, and that is likely what keeps it from being expressed in the courtroom. Prosecutors care more about their record than they care about ruining people’s lives.

Anonymous Coward says:

The problem with all of these sciences is that they’re mostly subjective.

As I’ve said before: There are two flavors of ‘science’: One is the fundamental belief that rational thinking is useful (Things like "the scientific method" are a direct outcropping of that).

The other is a religious, fanatical at time, belief in the things said by people who are called (either by themselves or others) "scientist".

This comment has been deemed insightful by the community.
Upstream (profile) says:

Old, but still outrageous, news

As the article points out:

In 2009, the National Research Council condemned blood pattern analysis as "more subjective than scientific."

And it was old news even then. Many people have been shouting this to anyone who will listen for several decades, probably most notably Radley Balko. But, as the article also points out, no one seems to be listening, especially not the criminal legal system.

It is also worth noting that, with the exception of DNA testing, all of the forensic techniques in use by the criminal legal system were developed either by, or for, people associated with the prosecution side of that system, specifically for the purpose of helping the prosecution secure convictions. These forensic techniques were not developed for, and are not used for, ascertaining the truth.

The accuracy, reliability, and evidentiary value of forensic evidence are almost always grossly overstated to juries, and many jurors have been heavily influenced by the CSI effect, which results in their giving grossly excessive weight to the evidentiary value of said forensic evidence.

Correcting the situation will be very difficult, because, just as in the drug war, there has arisen a very large law enforcement / prison / industrial complex around the practice of this forensic hocus-pocus. That makes for a lot of powerful and influential people all trying to maintain the status quo.

Whoever says:

Re: Old, but still outrageous, news

It is also worth noting that, with the exception of DNA testing, all of the forensic techniques in use by the criminal legal system were developed either by, or for, people associated with the prosecution side of that system,

I think that you will find that the initial development of DNA science in a criminal case was done by a team working with the police in the UK. Fortunately for the first person accused of the crime, the senior policeman in charge of the investigation followed through on the DNA analysis even though they had a suspect in custody: the DNA exonerated him. The culprit was eventually caught after someone reported overhearing a conversation in which someone told a friend that they gave a DNA sample for someone else (there was a mass DNA collection effort in the region of the country where the perpetrator was believed to live or work).

Anonmylous says:

This is what happens when prosecutors and investigators turn a statistical tool into "hard evidence" because they can’t come up with anything else. "Beyond reasonable doubt" is the bar, and tools like BPA should be used for eliminating suspects, not confirming them. Perhaps even going so far as to giving reason for a warrant to search further. But not for arrest and certainly not for prosecution. It was always a ballpark tool, as the amount of force someone who weighs 120lbs soaking wet and someone who weighs 240lbs dry as a bone, are in fact different, letting you eliminate suspects based on the physical ability needed to cause a specific impact.

Prosecutors lead the charge in turning these tools into evidence rather than investigative leads. Police simply follow suit and this leads to lazy policework. Policework is difficult for a reason, our Constitution is geared towards making it so, as it should be. To prevent a tyrannical government.

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