2.4 Example: fibre interpretation

Fibre interpretation is a term used for interpreting forensic findings related to textile fibres which are shed from textile materials (such as an item of clothing) as a result of contact with another textile material or surface.

Fibres can be transferred via direct and indirect contact. Direct contact means that the transfer was caused by physical contact between a textile material which sheds the fibre and a surface on which the fibre is found. Indirect contact means that the transfer occurred despite there being no physical contact between the textile material which shed the fibre and the surface on which the fibre is found. This can happen when fibres are transferred in a chain of one or more direct contacts with other surfaces so that the start and end materials in the chain never have direct contact with each other.

Fibres that are recovered from a surface or textile material of interest to an investigation, known as questioned samples, can be compared to fibres which are taken from a suspected textile material of origin, known as reference samples. The comparison aims to distinguish between the questioned and reference samples using properties of the fibres such as their material, size, shape, and colour.

Fibres can be useful for forensic interpretation because they can help to discriminate between hypotheses of physical contact between persons of interest in an investigation. They give the fact finder information about a possible activity having taken place perhaps in a specific location.

In this example, we consider some generic uncertainties encountered by a fibre examiner when interpreting the implications of recovered (or absent) fibres. After a suspect has been identified and taken to trial, the primary uncertainty for the court to consider is whether or not the suspect committed the crime in question. This is epistemic uncertainty: the suspect either did or did not commit the crime but the fact finder does not know which is true prior to seeing evidence (although innocence is presumed). Depending on the case circumstances, the expert’s fibre interpretation alone will likely not be sufficient to reduce the fact finder’s uncertainty about whether the suspect committed the crime or not. However, it may be helpful in reducing the fact finder’s uncertainty about other related hypotheses, for example, whether or not the suspect was present at the crime scene. This could be useful for the fact finder to combine with other evidence.

Suppose that the expert is tasked with interpreting fibres recovered from the suspect’s clothing (the reference sample) and fibres recovered from the crime scene (the questioned sample). There are many potential objects of uncertainty for the fibre examiner to address in this general situation, and in practice the examiner is usually directed as to which objects of uncertainty to address. In this example, we select the object of uncertainty to be whether or not the suspect was at the crime scene. This is an example of epistemic uncertainty.

  • Object of uncertainty: whether or not the suspect was at the crime scene.
  • Reason for uncertainty:
    • two credible alternative events presented by prosecution and defence,
    • variability in fibre recovery, transfer, and persistence,
    • limited relevant scientific studies.
  • Level of uncertainty for each of the above reasons:
    • direct,
    • direct,
    • indirect.
  • Magnitude of uncertainty: the expert can take into account relevant case circumstances as well as what is known about the variability in fibre recovery, transfer, and persistence to determine, for example, that
    • out of every 1000 similar case circumstances they would expect 200 of them to yield the questioned sample of fibres if the suspect had truly been at the crime scene,
    • out of every 1000 similar case circumstances they would expect only 2 to yield the questioned sample of fibres if the suspect had truly not been at the crime scene,
    • limited relevant scientific studies present uncertainty that is difficult to quantify.
  • Communication of uncertainty: the expert can compare the quantified uncertainty and caveat the qualitative uncertainty to state that
    • it is 100 times more likely (200 out of 1000 times compared to 2 out of 1000 times) that the fibre evidence would have been observed if the suspect had been at the crime scene rather than if they had not,
    • there are limitations to the available scientific studies, and state them.

The fact finder can use this communication, as well as other evidence, to make a decision as to whether they believe the suspect was at the crime scene or not.

In the example above, we only focussed on one possible object of uncertainty. In reality, there are many more objects that the expert has to consider and combine when interpreting forensic findings. The reasons for uncertainty for each of these objects will be case-dependent and this is further complicated by the complex nature of real-life criminal activities. This makes assessing uncertainty challenging. The expert might reasonably ignore some uncertainties to make this task more feasible. Many possible uncertainties can be excluded by only considering the sequence of events put forth by the prosecution and defence.