Bloodstain pattern analysis

Bloodstain pattern analysis (BPA) is a controversial subjective practice that consists of the study and analysis of bloodstains at a known or suspected crime scene. This is done with the purpose of drawing inferences about the nature, timing and other details of the crime. It is used mostly to study homicide or other violent crimes in which blood is present and is claimed to help in crime scene reconstruction. Since the late 1950s, BPA experts have claimed to be able to use biology, physics (fluid dynamics), and mathematical calculations to reconstruct with accuracy events at a crime scene, and these claims have been accepted by the criminal justice system in the US.

The validity of bloodstain pattern analysis has been questioned since the 1990s, and more recent studies cast significant doubt on its accuracy. A comprehensive 2009 National Academy of Sciences report concluded that "the uncertainties associated with bloodstain pattern analysis are enormous" and that purported bloodstain pattern experts' opinions are "more subjective than scientific." The report highlighted several incidents of blood spatter analysts overstating their qualifications and questioned the reliability of their methods. In 2021, the largest-to-date study on the accuracy of BPA was published, with results "show[ing] that [BPA conclusions] were often erroneous and often contradicted other analysts."