Investigators look at the clues left at the scene of a crime in order to reconstruct the event so they can understand what happened there. This is especially important when it comes to violent crimes, where evidence often points to suspects. When crimes involve a lot of blood, expert analysis of blood spatter patterns is used to draw conclusions as to the types of weapons, the sequence of events that occurred, and exactly how and why the blood wound up the way it did.
The History of BPA
This story begins in the basement of a New York home decades ago, where Herbert MacDonell put his own blood in a medicine dropper and dropped it onto a mirror in order to analyze the patterns that formed. A similar courtroom demonstration fascinated many, and blood spatter analysis was born. Since that time, conclusions have become generalized over a variety of surfaces, and courts across the country developed confidence in BPA analysis. However, in 2009 the National Academy of Sciences found that expert analysis was only a subjective conclusion, not a scientific one and that the reliability of blood spatter analysis was questionable. The belief that bloodstains can tell the facts behind a crime is no more than wishful thinking—and misguided at that.
What Do Analysts Claim?
The examination of the location, shapes, and patterns of blood found at a crime scene is all part of bloodstain pattern analysis (BPA). Analysts believe they can draw conclusions about what they see about the following:
- The way in which the wounds were caused;
- Which direction wounds came from;
- Where the blood came from;
- The locations of both the perpetrator(s) and victims(s) and whether the body was moved after the wounds occurred;
- Whether there was one or more perpetrators;
- How well witness statements match up with the evidence collected at the scene;
- Particulars as to what happened and importantly, what did not.
How Reliable is BPA?
The fact of the matter is that the error rate for conclusions based on BPA evidence is higher than you might hope. As much as prosecutors may want to rely on the analysis of their forensic “experts” when it comes to blood spatter evidence, the potential for mistakes is a serious issue. One study reported by the National Institute of Justice found that 11% of BPA conclusions turned out to be erroneous. Those odds are horrible: more than one case of BPA analysis will draw incorrect conclusions out of every 10 cases analyzed. Equally disconcerting is the fact that when any two analysts’ conclusions were compared, opinions did not agree in at least 8% of cases. Wouldn’t it be possible, then, to find an analyst to support any number of conclusions relative to a single crime scene? Continue reading