Summary of concepts Bloodstain Pattern Analysis

Publication | 11-10-2023

ASB

Academy Standards Board.

Bloodstain pattern

A grouping or distribution of bloodstains that indicates through regular or repetitive form, order, or arrangement the manner in which the pattern was deposited.

Context information management

Managing context information in forensic casework aims to minimise the task irrelevant information while maximising the task-relevant information that reaches the practitioner.

Evaluative reporting (Evaluative opinion)

Evaluative reporting evaluates the forensic findings in the light of at least one pair of propositions. It is based on a likelihood ratio and conforms to the principles of evaluation. Most of the time, evaluative reporting will follow from comparative examinations between material of unknown source and reference material from one or more potential source(s) and/or associated activities. An evaluative report is any forensic expert report containing an evaluative reporting section.

Expert elicitation

In science, engineering, and research, expert elicitation is the synthesis of opinions of authorities of a subject where there is uncertainty due to insufficient data or when such data is unattainable because of physical constraints or lack of resources.

Framework of circumstances

A summary of all of the case-specific information known to the expert about the
alleged offence and suspect(s) that is relevant to the assessment and
interpretation of the observations. This framework must always be regarded as
provisional because it depends on the evidence of others (police, crime scene
managers or crime scene investigators, pathologists, eye witnesses, victim,
suspect etc).

Investigative reporting (Investigative opinion)

Investigative reporting provides explanations for technical/factual findings. The
investigative approach is used when it is not possible to formulate a pair of competing propositions. This happens when there is insufficient background
(conditioning) information or when the investigators requested explanations
for findings at a scene and there is no obvious alternative. The absence of an
alternative proposition when for example one party makes “no comment” may
also lead to investigative reporting.

Likelyhood ratio (LR)

This is the ratio of two probabilities; the probability of the evidence given that the prosecution proposition is true divided by the probability of the evidence given that the alternative proposition is true. These probabilities are assigned on the basis of the scientist's expectation of the outcomes of the examinations, given that each of the propositions is true.

Objective features (of a bloodstain or pattern)

Observable and measurable physical features of bloodstains (e.g., size, location, distribution, shape, edge characteristics) used to distinguish between bloodstains and bloodstains pattern and used to classify them.

Proposition (hypothesis)

A formal hypothesis that is generated, in part, from the background information but may also depend upon the observations that have been made. In the context of a criminal trial there will most often be a pair of propositions — one representing the prosecution position, the other representing the defence's. Propositions are mutually exclusive (i.e. if one is true then the other must be false) and exhaustive (i.e. they cover all possibilities within the framework of circumstances). In the forensic context propositions contain the so-called forensically testable parts of a scenario that can then be expressed in hypotheses (propositions).

Scenario

Within the forensic context a scenario describes all events and/or activities that took place prior or during or after the crime.

Technical (factual) reporting

In most cases, technical reporting precedes investigative or evaluative reporting. In a strict sense, purely technical or factual reporting amounts to a descriptive account of findings. In certain situations, the descriptive statement of observations may lead to particular conclusions, such as a statement about the nature of particular physical matter, or - more formally - the assignment of an object to a class (i.e., classification). Technical reporting is often restricted to the results associated with the observations of items. It can involve the reporting of quantitative measure(s) of an attribute (such as weight or concentration) associated with the item. These measure(s) are generally reported together with some indications of their associated uncertainties (precision, accuracy of the technique). Examination methods and analytical sensitivities will often be major constituents of technical reports. Even though such reports may contain elements of statistical evaluation, they remain descriptive and do not constitute evaluative reports as defined in this document. A technical report does not involve a formal evaluation, under a pair of competing propositions, expressed in terms of a likelihood ratio.

Transposed conditional statement (prosecutor fallacy)

In legal contexts, a fallacious transposed conditional statement is one that equates (or, confuses) the probability of particular findings given a proposition with the probability of that proposition given these findings.