Author calls on lawyers to be alert to the implications of the choice of language in expert reports, and reports on recommended best practice from Canada in writing reports

“Vague and insignificant forms of speech, and abuse of language, have so long passed for mysteries of science; and hard and misapplied words, with little or no meaning, have, by prescription, such a right to be mistaken for deep learning and height of speculation, that it will not be easy to persuade either those who speak or those who hear them, that they are but the covers of ignorance, and hindrance of true knowledge.”
(An Essay Concerning Human Understanding, John Locke, 1690)

The common perception of the expert as the user of “vague and insignificant forms of speech” is not diminished when the lawyer is presented with expert reports that speak of STRs, PCR, THC (a drug), and TLC (a technique). At least the reader knows that they are unlikely to be expected to understand these and can seek guidance. But there is a far more insidious expert language that can give an impression of clarification, but in fact is more likely to obfuscate or, worse, mislead. Reports feature words or phrases that can leave lawyers either confused or convinced, sometimes wrongly on the latter, and culpably in the former.

Blinding with science

The choice of words can imply a meaning to a phrase. This may not be the intended meaning, or the whole story. Either way, the opportunity to comprehend and therefore possibly challenge what is being said can be lost. In the legal context that means the defence may be unaware of the potential challenges to the evidence. We deal frequently with cases where the defence lawyer has simply failed to understand the probative implications of some expert reports or specific parts of reports. This is not necessarily any fault of the lawyer, nor arguably of the expert. The expert has said nothing wrong, and in most cases has not attempted to hide anything. But even what to the expert are plain terms, may be anything but plain to others.

For example, a toxicologist in a drug-driving case reports: “specific analysis confirmed the presence of carboxy-tetrahydrocannabinol (carboxy-THC) at a concentration of approximately 40 ng/ml (nanograms per millilitre) of blood”. The defence lawyer may be forgiven for thinking there is a mountain to climb. When the next sentence says: “Carboxy-THC is pharmacologically inactive but is a breakdown product of cannabis”, the lawyer can now summarise the report as “blah blah cannabinol blah blood blah blah CANNABIS. Oh no!” A “defence” scientist would see “carboxy-THC, pharmacologically INACTIVE. Phew!”

The report did not intend to mislead, it just baffled. If the scientist reported that carboxy-THC, unlike THC, had no known effect on driving ability but merely showed that an unknown amount of cannabis had been consumed at some unknown time before the blood was taken, the lawyer might understand that this was no evidence at all of being under the influence of drugs at the relevant time – only measurement of the active compounds could do that. In that particular case the scientist chose to insert, in the technical section: “Cannabis may adversely affect a person’s ability to properly drive a mechanically propelled vehicle.” True, but not really germane if there was no evidence of cannabis at the relevant time.

There are a number of off-the-shelf phrases that find their way into expert reports and, regardless of the context, should raise a red flag with the lawyer. They should always be explored.

“Consistent with”

Perhaps the most common of these in forensic literature is the phrase “consistent with”, as in:

  • “consistent with his having been struck at least twice with a heavy blunt object or objects”;
  • “consistent with it making a forceful contact with a source of wet blood”;
  • “The mixture could be resolved to give partial profiles of the major and minor DNA contributors. These were consistent with the profiles of [A] and [B]”;
  • “The red mark on her chest was consistent with rough handling”;
  • “entirely consistent with the cause being a very superficial burn” (does this one mean “even more consistent with”?);

And this Wodehousean variation on the theme:

  • “He also states that the defendant’s version of events is not inconsistent with the post mortem finding.”

One recent report we received had 11 instances of the phrase – about one every three pages.

The adage comes in different forms. (An adage is defined in Wikipedia as “a short, but memorable saying, which holds some important fact of experience that is considered true by many people, or it has gained some credibility through its long use”! How apposite.)

“Cannot be excluded”

This phrase, becoming popular in DNA reports, appears as, for example:

  • “[A] and [B] cannot be excluded as being contributors of DNA to this mixed DNA profile”.
  • “[A] cannot be excluded as being the contributor of these minor DNA components.”

This is of course just a rehash of the “consistent with” form. It can be rephrased as “These minor components are consistent with [A] being the contributor of these minor DNA components.”

Many “marks and impressions” examiners also offer reports with statements such as: “I cannot exclude [item] as being the cause of this mark”. Given that no mark is a totally accurate reproduction of the source, the key question to be asked is “How different would the mark have to be to exclude this thing as the source?”

Probative value

So what is the problem with these statements? Surely the court needs to know whether the evidence supports some version of events. Of course it does. However, the phrases should be interpreted, and preferably stated, as “one of the possibilities”. Before evidence can be presented to a court it must conform to the jurisdiction’s rules on admissibility. Generally speaking, most jurisdictions will require that admissible evidence is “probative”: providing support to one side of a case or the other.

Probative value will be specific to a case rather than to a specific finding. Assessing the probative value is part of the evaluation of the evidence. The “consistent with” approach ignores two major questions that must be answered before the court can properly evaluate the probative value of this type of evidence: How many other stories does this evidence support? What is the weight of support for each?

Consider the problem of mixed DNA profiles: “The minor component was a partial DNA profile, consistent with DNA from at least one person being present.” Could there be two people? Without dealing with the detailed reasons here, the answer is unequivocally “Yes”. Three? Yes. There is simply no way of knowing which is true, from the evidence alone. It is perhaps more likely that there is one person than two, or two than three, but from the evidence, you have no way of knowing which is true.

Alternative explanations?

And therein lies the nub of the issue. The role of the expert is to evaluate the evidence, not the stories. The scientific approach is to consider the probability of the story given the evidence, not the other way around. The evidence is the scientific observation, that which the expert is being asked to assess because of their knowledge. This does not preclude the expert being asked to consider whether a story is possible. But they must consider it alongside the possibility of a different story, or stories. That there are other possibilities without a scientific means of discriminating among them should be made known to the court; it is for the court to consider the various possibilities:

  • If a wound is consistent with being caused by a 6” blade, then presumably it could also have been caused by a 12” blade that only penetrated half way, or an 18” blade that only went in one third of the way. The evidence is consistent with them all, and many more, and none of them can be excluded.
  • Even though an individual is a potential contributor to a DNA mixture, there are likely to be a large, unknown number of people in the population who could have contributed to the mixture.
  • If only some of the alleles (DNA sequences) from an individual are present in a mixture, can they be excluded? Some argue that if the alleles that they would “expect” to be there are not found, this is because the profile is not only mixed, but partial. They argue that the alleles are not detected because of degradation or being present at undetectable levels. The problem with this logic is that no one can then be excluded and the profile has no probative value at all.

The issue is not, does the evidence support this story, but, of all the stories the evidence supports, is this one the most likely?

Lessons from Canada

A Canadian commission of inquiry into miscarriages of justice (see has addressed these same issues and made some recommendations to minimise the risk of miscarriages arising because of flawed expert evidence. The commission identified many problems in the generation and presentation of scientific and medical evidence. Some of the recommendations are:

  • “Recommendation 8 – The use of appropriate forensic language

“The Centre of Forensic Sciences should endeavour to establish a policy for the use of certain uniform language which is not potentially misleading and which enhances understanding. This policy should draw upon the work done by forensic agencies or working groups elsewhere and the input of other stakeholders in the administration of criminal justice. This policy should be made public.

  • “Recommendation 9 – Specific language to be avoided by forensic scientists

“More specifically, certain language is demonstrably misleading in the context of certain forensic disciplines. The terms “match” and “consistent with” used in the context of forensic hair and fibre comparisons are examples of potentially misleading language. CFS employees should be instructed to avoid demonstrably misleading language.

  • “Recommendation 10 – Specific language to be adopted

“Certain language enhances understanding and more clearly reflects the limitations upon scientific findings. For example, some scientists state that an item “may or may not” have originated from a particular person or object. This language is preferable to a statement that an item “could have” originated from that person or object, not only because the limitations are clearer, but also because the same conclusion is expressed in more neutral terms.”

How long before we see these phrases become the central issue in a Scottish appeal?

Professor Allan Jamieson is Director of The Forensic Institute, Glasgow, t: 0141 202 0700, e:


Monthly seminars are held at The Forensic Institute, 50 Wellington Street, Glasgow

(all 5.30-7pm). Next events are:

  • Monday 8 October: Wound causation
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