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 obsrved. 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. research 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 2016, the texas forensic science commission reviewed cases that had used bloodstain pattern analysis, and consequently established that starting in 2019, bloodstain pattern analysts will need accreditation to testify as experts in texas courts. 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. relevant case histories a number of court cases are controversial due to their reliance on bloodstain pattern analysis: warren horinek a 1995 murder case against warren horinek was largely decided based on bloodstain evidence that has been hotly disputed after the fact. the police and the district attorney's office believed in horinek's innocence. the appointed attorneys for the prosecution found a bloodstain pattern analyst who testified that, rather than being a suicide as believed for a number of reasons by police, it was a murder due to the pattern of small blood flecks found on the accused, which according to the analyst had to have come from "high velocity" blood from a gunshot, rather than blood that simply got on him through his attempts to provide medical aid to the victim. other bloodstain pattern analysts have since disputed this claim and said that the bloodstains were consistent with medical aid. the original analyst has conceded that his claim is not as strong as he originally presented it as being, although he still believes in horinek's guilt. as of 2017, horinek remains in prison. david camm in the criminal case against david camm, who was tried three times for the murder of his family largely on the basis of blood spatter evidence, both prosecution and the defense used expert bloodstain pattern analysts to interpret the source of the approximately 8 drops of blood on his shirt. the prosecution's experts included tom bevel and rod englert, who testified that the stains were high-velocity impact spatter. paul kish, barton epstein, paulette sutton, barrie goetz, and stuart h. james testified for the defense that the stains were transferred from his shirt brushing against his daughter's hair. dr. robert shaler, founding director of the penn state forensic science program, decried blood spatter analysis as unreliable in the camm case. "the problem, in this case, is the number of stains minimal," he said. "i think you're really on the edge of reliability." all of the blood spatter analysts involved in the case are "experts" in the traditional sense. quoting shaler, "we have two opinions in this case. that, in essence, is a 50 percent error rate. " this would be ⁠an unacceptable level of reliability in a court case given that the perception of guilt beyond a reasonable doubt is what is required. 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. travis stay other times, bloodstain patterns from different causes can mimic each other. in the 2008 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. in popular culture sam tyler, the time-travelling detective in episode 3 of the bbc tv series life on mars, asks his colleague chris skelton to make a blood pattern analysis, quickly realizing by his puzzled expression that in 1973 such techniques are not known. serial killer dexter morgan of the dexter novels and showtime series is a blood spatter analyst for the fictitious miami metro police department. julie finlay is a blood spatter analyst on the cbs series csi: crime scene investigation.