People v. Blacknell, 2015 WL 6157479 (Ct. App. Cal. 2015)

Trial court reasoning for allowing the testimony: ” It found toolmark testimony had been admitted in California courts for many years and had not been subject to question. Further, reviewing the evidence presented at the hearing, the trial court found toolmark testimony was valid. It considered the “credibility” and “demeanor” of defendant’s experts and found their testimony did not disclose errors in identification procedures. It recognized these experts’ critique was the lack of a statistical foundation for making statements that casings and guns matched to a certain degree of probability. But it was concerned these experts had no experience actually performing toolmark identification. It further found Murdock and the prosecution witnesses credible on the manner in which toolmark analyses are done. Finally, it found the NAS reports, though raising criticisms of the toolmark field, explicitly disclaimed an intent to address the credibility of the field in court. When asked for clarification, the trial court explained it would not be limiting the nature of the statement of probability that an expert could make about a match, leaving it for cross examination to explore the strength of any statement of a probable match.”

Expert testimony in trial court: “He was questioned about the meaning of “practical” certainty or identity. He was questioned about the NAS reports and other criticisms of the field. He was questioned about when examiners reach opposite conclusions and about a lab in Detroit with high error rates. He conceded identifications to the practical exclusion of other guns were not based on statistical models or mathematics.”