What makes expert testimony admissible?

What actually makes expert evidence admissible in court? Or rather, what makes expert evidence inadmissible in court?

There are several things that need to be checked when an expert witness gives testimony, and this can often be uncovered through cross-examination of the witness by legal professionals (Davies & Beech, 2017). It must be identified accurate and have been interpreted correctly. Similarly, it must be checked whether any methodologies used are robust and valid for the current scenario. It must also be identified if there have been any conflicts of interest, perhaps by the people funding the research conducted or with anyone in the case itself. Finally, it must also be gathered whether the testimony is needed in the first place – does it actually lead to any benefit for the case that could not have been reached otherwise? If any of these points are not adequately met by an expert, then their testimony may be inadmissible.

These points are the justification behind the Frye/Daubert guidelines (1923, 1993 as cited in Cappellino, 2022) for identifying admissible evidence/testimony. These stipulate that there are several criteria which must be met in order to ensure testimony is needed and credible:

A The extent, quality and method used to obtain information

B The reliance on a statistical inference and the margin of error used

C The extent to which others with relevant expertise have reviewed it

D The concurrence of others in the field (or explanation of divergence)

E The consideration of context and other related facts

These need to be considered before the testimony is even given in court to try to measure that reliability. However, these criteria are generally used within the US rather than the UK. While we attempt to apply similar types of principles in the UK, we tend to find that these principles are often not upheld consistently, which can cause significant issues with expert testimony in UK courts (Ireland & Beaumont, 2015) – we will look at these pitfalls a little later.

One important process in deciding the admissibility of expert testimony is known as a voir dire.

Voir dire

A voir dire is essentially a 'trial within a trial' (Davies & Beech, 2017) and is a key means of identifying the admissibility of evidence and the credibility of experts. In such cases where evidence is questioned regarding its admissibility, or the credibility of an expert is called into question, a voir dire can be called whereby the main case is suspended and instead the evidence/expert is put on trial (this is done without a jury). The aim of the voir dire is to test and cross-examine the evidence/expert to identify its/their credibility. If it is deemed credible in a voir dire, it may be used in the main case, but if it is deemed non-credible, it becomes inadmissible in the main case.

In the following section we'll consider some of the issues with expert witnesses.