Bloodstain pattern analysis has been used informally for centuries, but the first modern study of blood stains was in 1895. Dr. Eduard Piotrowski of the University of Kraków published a paper titled "On the formation, form, direction, and spreading of blood stains after blunt trauma to the head." He conducted his first experiments on rabbits. A number of publications describing various aspects of blood stains were published, but his publication did not lead to a systematic analysis. LeMoyne Snyder's widely used book Homicide Investigation (first published in 1941 and updated occasionally through at least the 1970s) also briefly mentioned details that later bloodstain experts would expand upon (e.g., that blood dries at a relatively predictable rate; that arterial blood is a brighter red color than other blood; that bloodstains tend to fall in certain patterns based on the motion of an attacker and victim). A 1952 episode of the police procedural radio series Dragnet made reference to bloodstain pattern analysis to reconstruct a shooting incident.
The first formal bloodstain training course was given by MacDonell in 1973 in Jackson, Mississippi. MacDonell taught workshops on how to conduct bloodstain analysis, and the newly trained bloodstain analysts, who often had received as little as 40 hours of instruction, in turn would give expert testimony in court cases. In 1983, the International Association of Bloodstain Pattern Analysts was founded by a group of blood stain analysts to help develop the emerging field of bloodstain pattern analysis.
Starting in 1995, court cases where bloodstain analysts disagreed with each other raised concerns of the discipline's prediction's admissibility as evidence in court. In 2009, the National Academy of Sciences published an examination of forensic methods used in United States courts which harshly criticized both bloodstain pattern analysis and the credentials of the majority of the analysts and experts in the field. Judges have largely ignored the report's findings and continue to accept bloodstain pattern analysis as expert evidence.
Blood is composed of three components suspended in plasma: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).
When a blood vessel is damaged, the blood starts moving outside the circulatory system, following the laws of physics. If this leaking blood ever reaches the surface of the skin, it will be externalized and will likely come into contact with the person's surroundings. Some of the most common interactions are:
Since blood is 55% plasma (and 45% solid components suspended in plasma), it is an aqueous liquid and behaves as such. More specifically, it is a shear-thinning, non-Newtonian fluid. Here are its three main physical properties:
A bloodstain can present itself differently depending on the situation and the material on which it appears, and bloodstains may be hard to examine on porous surfaces such as fabric, and may be distorted. Bloodstain pattern analysts consider the angle of impact to determine its origin and the amount of force behind it; variations in external forces can cause satellite drops. A point of origin can be determined by finding what bloodstain analysts call the "area of convergence" for the blood droplets. To find this point of origin, the shape of the blood and the length are often taken into account and the stringing method is implemented. In the stringing method, blood drop paths are depicted as straight lines. Strings are placed at the bloodstain positions and pulled away from the surface to reconstruct the direction of impact. This direction is determined by the shape and orientation of the bloodstains. The point where most strings intersect is considered the estimated location of the blood source. There is also a method known as the tangent method. In this method, the blood drops' paths are seen as right-angled triangle hypotenuses. This works best for fast-moving drops with flat trajectories, but uncertainties in their curvature may lead to errors in determining the blood source's horizontal position."
Additionally, the angle of impact as well as other external factors such as the material on which the blood falls can change the shape and size of the blood. The point of impact can change the shape of the bloodstain. Bloodstains, instead of maintaining their original forms, may become elongated. In these cases, the blood may have a tail capable of indicating directionality. In order to find the angle of impact investigators measure the length and width of the blood droplet and use the formula
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{\displaystyle sin(A)=width/length}
. The (A) representing angle of impact.
Impact spatter is the most common bloodstain pattern type in a crime scene. It occurs when an object hits a source of blood. In impact blood spatter patterns, blood is often circular and not elongated. There are two types of impact spatter, back spatter and forward spatter. Back spatter occurs when blood is projected back at an attacker, while forward spatter is blood that exits directly from the victim's wound and projects onto nearby surfaces. The speed of the weapon used in the attack can cause changes in the size of blood spatter. The speed of the attack is classified into high, medium and low velocity attacks. High-velocity spatter (e.g., gunshot wounds) create small-sized droplets. High-velocity spatter usually travels 100 feet per second and creates blood droplets sized 1 millimeter or less. Medium-velocity spatter (e.g., blunt force trauma) is often made with a weapon and can create cast-off patterns. They are often made at between 5 and 25 feet/second the blood droplets ranging from 1 to 4 millimeters in length. Low-velocity spatters are usually created just as a result of blood dripping from the individual (i.e., gravity). They can also be referred to as passive or gravity bloodstains (bloodstain patterns that are formed under the influence of gravity), and are separated further into four categories: transfer/contact stains, flow artefacts, drop stains, and pooling. Transfer stains occur when two surfaces come into contact and at least one has blood on it, and it includes swipe and wipe patterns, which can give information regarding sequence of movement in some cases. Pooling occurs when the source of the bleeding remains static for a certain period of time, the blood continuously dripping in the same location and resulting in an important accumulation. If the individual who is actively bleeding moves while blood is dripping, the resulting pattern will allow for determination of direction and relative speed of movement at that time. Cast-off patterns are associated with impact spatter. These patterns arise from blood being ejected from a bloodied or bleeding object during its movement, commonly observed in incidents involving physical assaults or strikes. They are commonly observed on ceilings when objects are swung overhead, although they can potentially appear on any surface within the surrounding area. These patterns may be used to guess the direction of a weapon swing. In these cases, the length and the shape of the bloodstain patterns can help determine the speed of the swing. These patterns create elongated or elliptical shapes in blood when it hits the surface of an object. In 1895, Dr. Eduard Piotrowski's experiment showed that these patterns are often created after the second hit using the weapon. In some cases, void or shadow patterns can be observed. It is the result of a person or an object shielding an area from the blood coming toward it, and it is characterized by a clean area where bloodstains are expected. It can help determine if whatever intercepted the blood has been moved since the incident occurred.
The reliability of courtroom testimony by bloodstain pattern analysts has come under fire, particularly in the wake of a 2009 report by the National Academy of Sciences, which found the method of analysis to be "subjective rather than scientific", and that it involved an "enormous" degree of uncertainty. In addition to concerns over methodology, the report criticized the lack of proper certification requirements for analysts and an emphasis on "experience over scientific foundations". Many bloodstain pattern analysts have testified in court as experts despite having received training only in the form of a 40-hour course taught independently by MacDonell or one of his students, without institutional accreditation or minimum educational requirements. Even with proper training and methods, there are still many times where reputable analysts disagree on their findings, which calls into question the reliability of their conclusions and its value as evidence in court.
There is very little empirical evidence to support the use of blood spatter analysis in court or any other aspect of the legal system. While certain aspects of bloodstain pattern analysis, such as methods for determining the impact speeds of splattered blood, are supported by scientific studies, some analysts go well beyond what is verifiable. In addition to problems with the underlying scientific validity of the method, the circumstances of bloodstain pattern analyses, which are often conducted at the behest of either the prosecution or the defense in a court case, can introduce confirmation bias into the analyst's assessment.
In 2021, the largest study to date on the accuracy of bloodstain pattern analysis was published in Forensic Science International. The study was based on 33,005 multiple-choice responses and 1760 short text responses, by 75 practicing bloodstain pattern analysts on 192 bloodstain patterns selected to be representative of casework, and stated:
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. The disagreements with respect to the meaning and usage of BPA terminology and classifications suggest a need for improved standards. Both semantic differences and contradictory interpretations contributed to errors and disagreements, which could have serious implications if they occurred in casework.
A number of court cases are controversial due to their reliance on bloodstain pattern analysis:
Further complicating matters was the testimony of Rob Stites, who testified for the prosecution as an expert blood spatter analyst. It was later uncovered that he had no training and his credentials were fabrications by the prosecutor. His testimony that the blood on Camm's shirt was high-velocity impact spatter aided in the conviction of David Camm. Dr. Shaler pointed out that one limitation of blood spatter analysis testimony is that "you do not have the supporting underlying science" to back up your conclusions. When Stites testified, the jury had no way of knowing that he was not the expert that he purported to be. Even among the expert witnesses, it is unknown which set of experts interpreted the stains accurately as there is no objective way of determining which bloodstain pattern analyst has applied the science correctly.
Other times, bloodstain patterns from different causes can mimic each other. In the 2008 North Dakota trial of Travis Stay for the murder of Joel Lovelien, prosecution witness Terry Laber testified that the blood spatter on Stay's clothing came from blows to Lovelien during a fist fight. After a review of the evidence by Paul Kish, another bloodstain pattern analyst, Laber reviewed the report submitted by Kish and revised his findings to include the possibility that the blood came from expiration by Lovelien.
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