Anonymised Summary of the Opinion

Factual and Procedural Background

This is an appeal under section 74(1) of the Criminal Procedure (Scotland) Act 1995 by the Appellant against a decision at an evidential Preliminary Hearing (PH) refusing her preliminary issue minute which objected to the admissibility of an expert report. The opinion was delivered by Judge Dorrian.

In substance, the Appellant is indicted for trial at the High Court for, inter alia, the murder of her partner. The Deceased died of a stab wound through the heart on 26 June 2022 at an address in The City. The Deceased and the Appellant were alone at the address when the death occurred; the Appellant was pregnant with the Deceased's child.

The Appellant provided two statements to the Police (one shortly after the incident and a second three days later). Broadly paraphrased, both statements describe the Deceased grabbing the Appellant during a struggle, wrapping his arms around her with his arms crossed in front of her, holding a knife in one hand and the sheath in the other, the Appellant freeing herself after pushing her pelvis back, and only afterwards seeing blood on the Deceased. In the statements the Appellant did not describe how the knife came to penetrate the Deceased's chest and did not say which hand held the knife.

The Police instructed an Expert Ergonomist to prepare a biomechanical report based on a range of material disclosed to him, which he described as including the Appellant's two statements, a statement by a friend of the Appellant, the post-mortem report, an analysis of the sharpness of the knife, and crime-scene photographs. The Expert also relied upon a demonstration reportedly given by the Appellant to police officers (the Expert was not present for that demonstration).

The Expert created computer simulations said to follow the laws of physics and biomechanics and to have been run many thousands of times over a range of variables (speed, trajectory, energy, etc.). His report concluded that "the probability that [the Appellant's] account can be supported by the biomechanical evidence is much less than 0.25%."

At the PH the temporary judge refused the Appellant's minute (i.e. did not exclude the Expert's evidence). The Appellant appealed that refusal to the Appeal Court.

Legal Issues Presented

1. Whether the proposed expert evidence comprised of computer simulations and a numerical probability statement was admissible at trial.
2. Whether that evidence improperly usurped the jury's role by advancing a purported statistical assessment of the credibility or reliability of the Appellant's account.
3. Whether the Expert's analysis had a sufficiently reliable factual foundation (including admissibility and selection of the Appellant's statements, reliance on a reported demonstration, and the many assumptions made about forces, trajectories, relative positions, and which hand held the knife).
4. Whether the Expert possessed the requisite scope of expertise (in biomechanics and in the use and presentation of probability/statistics) to give the evidence he offered.
5. Whether the evidence was "necessary" for the jury (the necessity requirement: expert evidence is admissible only where it assists on matters outside ordinary juror experience).

Arguments of the Parties

Appellant's Arguments

• The proposed expert evidence advanced a statistical assessment of the likelihood of the Appellant's account and thereby usurped the jury's exclusive function to assess credibility and reliability; the dynamics of a physical struggle are within ordinary juror experience and not a proper subject for expert evidence (authorities cited by the Appellant were relied upon in the PH).
• The modelling was based on hearsay and a set of untested assumptions and therefore lacked a sound factual basis. The Expert himself acknowledged that "considerable approximation is inevitable."
• The evidence was collateral and would distract from central issues; presenting a numerical estimate of probability risked establishing a new and inappropriate focus on percentage assessments of "reasonable doubt." The analogy to road-traffic reconstruction was said to be inapt.
• The Expert did not meet the criteria to be treated as a skilled witness for the task proposed; ergonomics, as a discipline, traditionally addresses workplace design and safety and its application to the dynamics of a fighting episode in a homicide context was unprecedented.

Respondent's (Crown's) Arguments

• The Crown required evidence to rebut the Appellant's account that the Deceased had accidentally inflicted the fatal wound on himself, and evidence from an expert in biomechanics/ergonomics would assist the jury in a subject outside ordinary experience.
• The Expert could draw inferences from the primary evidence that an ordinary juror could not; it is common and permissible for an account given to police to be addressed by a skilled witness for comment.
• While an expert must not usurp the jury's function, a skilled witness may address the ultimate issue in appropriate terms; the Expert had explained his method, assumptions and limitations and the temporary judge was entitled to conclude that the Expert was admissible for that purpose (reliance was placed on civil authority on expert scope in that context).
• The Expert's disciplinary basis was a recognised academic field; deficiencies in the factual basis or methodology could be explored in cross-examination or by calling a competing expert.

Table of Precedents Cited

Court's Reasoning and Analysis

The court undertook a structured analysis addressing: the scope of the Expert's competence; the admissibility and selection of the Appellant's statements; the factual foundations for the Expert's simulations; the relevance and necessity of the evidence; and the form in which the Expert expressed his conclusions.

Scope of expertise. The Expert was a consultant ergonomist and asserted a competence in biomechanics. The court observed that the extent to which biomechanics is a subset of ergonomics, and the Expert's experience in probability and statistics, were not satisfactorily explored in evidence. The court did not, however, reject the evidence on that technical basis alone; it proceeded to consider more fundamental defects.

Admissibility of the Appellant's statements. The court noted the Appellant's two police statements: the first given shortly after the event and the second three days later. The first might be treated as part of initial inquiries (and so its being given without a formal caution was of less consequence); the second statement was used by the Expert as the factual basis for his modelling. The court emphasised that the Expert relied only on the second statement and had not taken account of relevant differences between the two statements, which could materially affect positions, forces and movements modelled.

Factual foundation for the Expert's conclusions. Critically, in neither statement did the Appellant explain how the knife came to penetrate the Deceased's sternum; her account described only the struggle and that she had freed herself and later saw blood. The Expert therefore sought to extrapolate from the struggle to the energy transferred to the Deceased's arm and the resulting motion of the knife. The court identified multiple relevant unknowns which the Expert either omitted or treated as assumptions:

• There was no reliable evidence about relative heights, builds, strength or muscle development of the parties, yet these are central to force and acceleration assumptions; the Expert treated participants as equal when they were not.
• The Expert relied on a reported demonstration that he did not witness; the source and detail of that demonstration were not specified in the report or in evidence.
• Key variables were unknown or assumed without verification: which hand held the knife (the Deceased was described as ambidextrous), the path (straight or curved) and speed of the knife, whether restraining force was constant or varied, and whether any release was sudden.
• The simulation proceeded on the hypothesis that a "considerable force" restraining the Appellant was "suddenly released," causing the Deceased's arm to accelerate uncontrollably along a curved path; the court characterised these crucial premises as speculative.

Validity and probative value of the simulations. Although the Expert said he ran many thousands of simulations across a wide range of variables and arrived at a numerical conclusion (less than 0.25% probability that the Appellant's account was mechanically consistent), the court concluded that there was no sufficiently reliable, clear or specific factual basis to give those simulations probative value. The court contrasted this with legitimate reconstruction evidence in road-traffic cases, which is often grounded in objectively verifiable measurements; no comparable objective factual base existed here.

Expression of conclusions and use of probability. The court was concerned by the way the Expert expressed his conclusion as a very small numerical probability. The Expert was not a statistician, and the methodology by which his many simulations led to the stated probability was not explained in evidence. The court noted the risk that such numerical statements would complicate and confuse the jury's task and could lead to requests for further expert statistical evidence. The court also accepted that presentation of probabilities in legal proceedings is a delicate area prone to misunderstanding.

Additional technical concerns. The court pointed to apparent errors or unexplained measures in the report (for example, a figure used in formulae for an anthropometric measurement which diverged markedly from the actual measured lengths produced in evidence). Such issues raised the prospect that the report's calculations were undermined by mistaken inputs.

Necessity and usurpation. The court emphasised the necessity requirement for expert evidence in criminal trials: the jury only requires expert assistance where the subject matter falls outside ordinary knowledge or experience. The Appellant's anticipated evidence described a human interaction — a physical struggle — that the court considered well within the competence of an ordinary juror. The Expert's probabilistic conclusion, by effectively placing a percentage on the likelihood of the Appellant's account, risked usurping the jury's role in assessing credibility and reliability. For these reasons, and because of the speculative factual basis, the court concluded the evidence was not admissible.

Having considered these matters, the court concluded that the temporary judge had misdirected himself by failing to address adequately whether there was a sufficiently satisfactory factual basis for the evidence and whether the application of biomechanics in this context was appropriate; accordingly the temporary judge's decision to admit the evidence was wrong.

Holding and Implications

APPEAL ALLOWED — THE PROPOSED BIOMECHANICAL/ERGONOMICS EXPERT EVIDENCE IS RULED INADMISSIBLE.

Direct effect: the Expert's report and the computer-simulation evidence (including the numerical probability statement) are excluded and may not be adduced at the forthcoming trial. The court held that, on the record before it, the evidence lacked a sufficiently reliable factual underpinning and was unnecessary to assist the jury.

Broader implications: the opinion does not purport to establish a new general rule of law; it applies established principles concerning the role and limits of expert evidence in criminal trials (necessity, reliable factual basis, and avoidance of usurping the jury). The court emphasised the need for a demonstrably adequate factual foundation and methodological transparency before allowing biomechanical simulations and probabilistic conclusions of this kind.