The logic of forensic pathology opinion

Evaluating evidence and providing opinions are at the heart of forensic science, and forensic experts are expected to provide opinions that are based on logically sound and transparent scientific reasoning, and that honour the boundaries of their area of expertise. In order to meet these objectives, many fields of science explicitly apply Bayes' theorem, which describes the logically correct way to update probabilities on the basis of observations. Making a distinction between 'investigative' and evaluative' modes of operating helps to implement the theorem into daily casework. Use of these principles promotes the logic and transparency of the reasoning that leads to expert's opinion and helps the expert to stay within her remit. Despite these important benefits, forensic pathology seems slow to adopt these principles. In this article, we explore this issue and suggest a way forward. We start with a short introduction to Bayes' theorem and its benefits, followed by a discussion of why its application is actually second nature to medical practitioners. We then discuss the difference between investigative and evaluative opinions, and how they enable the forensic pathologist to reconcile Bayes' theorem with the different phases of a forensic investigation. Throughout the text, practical examples illustrate the various ways in which the logically correct way of evidence interpretation can be implemented, and how it may help the forensic pathologist to provide an appropriate and relevant opinion.

Homicides Disguised as Staged Suicides

Determining the correct cause and manner of death are among the chief responsibilities of the forensic pathologist. When confronted with a case having a questionable manner of death, it is important for scene evidence, witness statements, and forensic autopsy findings to be thoroughly reviewed before certifying the manner of death. Herein three gunshot wound cases are presented which initially were suspected to be suicides but were instead determined to be homicides through full investigations with correlation to autopsy findings.

Forensic Pathologist Consensus in the Interpretation of Photographs of Patterned Injuries of the Skin

Forensic pathologists are commonly asked to evaluate injuries on the basis of photographs. Members of the National Association of Medical Examiners were asked to participate in an online survey in which they were presented with 68 patterned injuries of the skin and asked to make a diagnosis ranging from very general (e.g., "blunt" vs. "sharp") to specific (e.g., "baton blow"). This was not the case. Consensus for general diagnosis averaged 0.77 and 0.72 for specific diagnosis. While there was a strong correlation between consensus and confidence in aggregate, individual correlations were poor. Consensus diagnosis was inversely correlated with age, and positively correlated with jurisdictional size, medical degree, and whether or not the respondent was actively performing autopsies as a job function. A subsequent survey is exploring possible reasons for lack of consensus in low-consensus questions. The high correlation between confidence and consensus at the aggregate level and low correlation at the individual level may have implications for quality assurance protocols.

Manner Determination in Forensic Pathology

A traditional function of forensic pathologists in the United States is the determination of cause and manner of death. The formal certification of manner of death is an American innovation of the early 20th century. Many countries do not follow this tradition. While originally established for vital statistics and public health purposes, the use of manner has grown to include increased legal, social, and business functions. The expanded role of manner determination has led to controversy regarding its true function, who should do it, and if it should be done at all. Much of the controversy regarding manner determination is due to inherent ambiguities that arise primarily from the limited number of categories and fundamental epistemic issues. Much of the heterogeneity in manner determination is structural and not a matter of error. Other criticisms come from those who focus on the legal implications of manner determination. Some have claimed that the use of circumstantial information in manner determination constitutes cognitive bias. Others suggest that manner determination is not a medical issue at all. A response is that the integration of circumstances and history with physical examination and laboratory findings represents the classic paradigm of medical diagnosis, and reinforces rather than opposes recognition that this is the practice of medicine. Transferring the task of manner determination to another agency would do nothing to improve accuracy, timeliness, or function. Other candidates to perform this function would be successful only to the degree that they were trained and function as forensic pathologists.

Intent in Manner Determination

Manner of death determination is a basic and traditional task for medical examiners. Though it has a history spanning centuries, many facets of manner determination continue to be problematic. Few issues are more vexing than the difficulties in determining intent. A survey of members of the National Association of Medical Examiners was performed to determine current practice and attitudes about the use of intent in manner determination. There were 168 completed responses, representing an approximately 20% response rate. The results reveal significant variation. While the concept of “volition” as distinct from “intent” has been common for over a decade, less than 60% of respondents made the distinction in their practice. There was also wide variation in the degree of certainty practitioners noted they used when determining manner with peaks at between 51-60% and 95-99%. Some of this represents real uncertainty in manner determination and some represents cultural and regional differences in practice. The variation and uncertainty in these determinations has led some to suggest that this role be abandoned. However, the statistical, policy, and cultural need for these determinations dictate that the function will be performed even if abandoned by the medical examiner community. Medical examiners are uniquely positioned and trained to fill this need, and should continue to do so.

Critical Appraisal of Scientific Literature

Despite the relatively small size of our profession, there is abundant scientific literature within the field of forensic pathology. The written word does not imply truth, however, and it is vitally important that the forensic pathologist approach the literature with a critical eye and understand how to evaluate a paper for oneself and come to a conclusion regarding its validity and the level of evidence it provides. The purpose of this paper is to provide a basic outline of the concepts important to critically reviewing a scientific paper; not so much as a peer reviewer, but rather as a practicing medical consumer of information. It will outline the key steps in approaching and reading a scientific study: identifying the type of study the paper represents, describing the process of critically reading the paper, assessing its scientific quality, describing various types of errors and where to look for them within the paper, and assessing the level of evidence provided in the study.