One of the strangest things I have heard raised in argument against the logical approach is that use of the approach means the expert’s testimony will end up impinging upon the authority of the Court.1 I have heard this a few times recently. I find this particularly troubling because it has come from lawyers. Unfortunately, this has always happened in circumstances where I could not actually discuss the matter with them.

As an objection to the logical approach, this is the most unexpected thing I have ever heard, without a doubt. In reality, proper application of the logical approach is one of the few ways to ensure that this issue will not happen.

To clarify, it is important to first understand the concept of “usurping the role of the Court” which means, in essence, to improperly influence the court’s procedures and decision-making, often by speaking inappropriately to or about the ultimate issue. Or, in other words, to impinge on the Court’s authority to make decisions about the ultimate issue, or ‘what happened’. To be sure, there is a legitimate concern that this could be a problem, particularly when the court is listening to an expert. As a result, the concept has been discussed literally for years and it is not a new concern.

In fact, it can be found in various codes and directives regarding expert evidence. Indeed, Justice Sopinka noted this precise issue in the 1994 R. v. Mohan ruling when he stated, in part, “There is also a concern inherent in the application of this criterion that experts not be permitted to usurp the functions of the trier of fact.” 

This is a legitimate concern because an ‘expert’ is often seen as being all-knowledgeable and infallible, even though both such things are impossible. People tend to view an expert’s opinion as something deserving of special consideration and respect. Now, this is true, if only because expert witnesses are allowed to express their opinion about things, unlike other witnesses. That alone makes them different from other witnesses. But it is wrong to think experts are perfect and infallible. That part is not true. 

Based on this mistaken belief it is possible that an examiner’s opinion might 1) be given too much weight, 2) replace or over-ride the opinion of the trier/jury, 3) be misused in a manner that distorts the fact-finding process, and/or 4) be confusing to the trier/jury.2 To be sure, this has always been the case and it is a definite concern when speaking about the approach traditionally taken by examiners in doing their evaluations and reporting. This is because experts have traditionally expressed opinions in terms of ‘what happened’. That is, their opinion literally spoke to the probability of events that may have happened to cause the evidence.

With that type of approach such concerns are clearly legitimate.

I have discussed this exact point with many examiners. They tend to downplay or dismiss such concerns saying something like, “it isn’t a concern because the trier can just dis-regard my evidence, if they want. Just because I’m saying X doesn’t mean the trier has to accept that at all.”

That’s true enough. A judge or jury can ignore evidence if they feel justified in doing so. But doesn’t that miss the point? First, don’t examiners want the trier to understand and use their information properly? Second, and more critically, we have no idea how our information will be used by the trier. We cannot anticipate how the trier will interpret our evidence so we should be very careful to avoid even the potential of usurping the role of the trier. And, we must always remember that this is evidence coming from an ‘expert’ so dismissing or ignoring it may be very difficult.

Now, in contrast to that traditional approach, the logical approach takes a different tack and, as a result, has precisely the opposite effect. Proper application of the approach goes a long way to help ensure that the examiner avoids even the potential to usurp the court’s authority.

It is a conundrum to me that lawyers, trained in matters of both law and logic, think that the logical approach is a concern in this regard. This misunderstanding is very problematic and speaks volumes about the state of knowledge and awareness with respect to the logical approach and how it actually works. 

Reality is exactly the opposite of what they think it is.  

I find it especially hard to understand in light of the vast literature that has been written about the logical approach in the last couple of decades.3 However, in fairness, people (including myself many years ago) may find it difficult to see and understand the distinction between the way things have traditionally been done and the way that proper logic dictates things be done.

For one thing, nobody wants to believe that their way of doing things is logically fallible or unwarranted. For another, people (e.g., lawyers and some judges) want the expert to speak directly to the issue at hand. In general, it makes sense that they should be able to do that. After all, they are an expert and people express things in that manner all the time in their daily lives. The problem is that the knowledge and information the expert possesses is insufficient for that purpose. 

The important thing to understand is that, even though examiners do not possess the requisite information to express such opinions, they can definitely assist another party in that party’s efforts toward that goal.  

I suspect that some of the confusion may come from mistakenly equating the logical approach and Bayes Theorem, in full. The two are NOT the same as I’ve discussed at length elsewhere. Bayes Theorem is a logically coherent process for belief updating and, as such, it might be a good way to conduct something like a trial. A trial, after all, is the ultimate belief updating process.4 The key, however, is that the belief being updated is not that of the expert; it is that of the trier.

To that end, the expert’s role within the logical approach is very limited and constrained. The expert (simply) assists the trier (judge and/or jury) in their determinations by explaining what domain-specific evidence means in a specified context. They do this by speaking solely to the evidence and never the propositions.

As a result, there is absolutely NO way this process can usurp the authority of the court because no opinion is ever given in terms of the probability of the propositions of interest. 

Another issue might be the setting of propositions as a concern for those lawyers. The setting of propositions is a challenge and a legitimate concern but, as discussed elsewhere, that is something the expert should, or would, not do if it were not the only viable option in some cases. 

Perhaps they don’t like the idea that the logical approach ‘requires’ two competing propositions to conduct a proper evaluation. Some lawyers like to point out that, in many legal systems, the defense need not hold or present a ‘position’ at all. That argument misses a couple of key things: first, there is always a counter-position that can be considered (the negation, if nothing else, even if that’s not the ‘best’ option for the defense); second, all traditional evaluations incorporate a counter-position even though it usually remains hidden or masked. I am, by no means, the first to point these things out.

A set of competing propositions is key to proper, balanced evaluation of evidence so they must be set or declared. If this is done by the examiner, then the set used must be declared so everyone knows what was done (and provide alternatives, if something else is preferred). Proper disclosure is a key aspect of the logical approach and should mitigate any issue with this completely.

A further complication comes from the fact that people ‘automatically’ transpose the conditional all the time. All of us do this regularly in our daily lives, but, in that context, we are the decision maker. We observe and evaluate constantly and then make some ‘decision’ based on our belief about what it means, with little or no conscious effort. That’s fine for routine ‘decisions’ made by people in their daily lives and, as a rule, things work quite well even though it can result in unexpected and fallacious outcomes from time to time. 

It is important to understand that forensic experts have historically been trained to do the same type of thing in their work. They are literally taught that certain observations mean that something ‘must’ have happened. While that may be true in some rare situations, it is not generally a sound approach because it ignores uncertainty about the relationship between evidence and cause. Such things are probabilistic, not definitive, in nature.

Additionally, lawyers do this all the time. That’s okay because their role is to present an argument; some reasoned story about what happened to cause an outcome, with sufficient proof to make a court accept their argument over some other. To that end, they must take all the information and try to create that story which, necessarily, speaks directly to ‘what happened’.

For them, doing so is not a problem because 1) it is literally their role, and 2) they have more complete knowledge of the situation. As such, they are in a position to consolidate all of the relevant information when formulating their belief. They can also apply whatever personal criteria they like to form their opinion so that it can then be presented to the court as an argument for their side of the matter. Of course, that process might entail a number of challenges in terms of combining information from various sources (i.e., evidence) to form the final story. Strictly speaking, those are concerns for the lawyers and the trier to address. The expert has no control over such things, beyond trying to ensure their own evidence is properly understood.

Similarly, judges must reach some decision in order to express their final opinion in a matter. In essence, the trier must listen to, and assess, the information provided by various sources (i.e., witnesses) and then determine ‘what happened’ as the ‘cause’ of the overall evidence. They will, of course, listen to the lawyers who frame that information in the best possible way for a particular side. Ultimately, the trier must consolidate all of the relevant information and apply some decision criteria or heuristic. Again, this is not a problem because 1) it is the judge’s role, and 2) they have access to all the necessary information, even more so than any lawyer.  

Experts are NOT in the same position. We provide limited and discrete information about one aspect of the matter. That information relates, and is limited, to issues within the scope of our expertise. As such, transposition that leads to a statement about the proposition(s) (i.e., what actually happened) is never warranted or justified. If an expert adheres to the principles and processes embodied in the logical approach, they will not express an opinion that can be viewed as usurping the authority of the court. They will, in fact, never make a statement about the truth/falsity/probability/likelihood of any proposition.

Their opinion should be limited to a statement, or statements, about the evidence in terms of each of the relevant propositions, if each of those were true. The evidence, therefore, provides support for one or the other (or both, to varying degree) of the competing propositions. 

Another complication is the lack of standardized wording for opinions. Not that there are no guidelines or standards. Just that the choice of the opinion wording is subject to discussion/debate. My preference is that of a concordance scale recommended by ENFSI. That scale ties the verbal opinion to a unambiguous level of relative belief about the propositions.

Admittedly, at this point in time, there is an open question as to whether or not a given method or examiner can use such a scale reliably and accurately. However, while important, that point is not an argument against using the logical approach. After all, there is far less standardization of opinion scales used with the traditional approach.

Finally, when critiquing the logical approach people rarely bother to think about the alternative that might be used, should the logical approach not be applied. They generally proceed under the assumption that the alternative is fine. This post, I hope, has pointed out a few of the ways that is simply not true.

Various judicial rulings critical of the logical approach have been flawed in this manner, even though the Court in those rulings attempted to be logical and sensible in their critique. They don’t seem to appreciate the fact that sticking with the status quo means the continued use of a flawed and illogical approach that leads, almost certainly, to the effect they wish to avoid.

To put it bluntly, if one does not follow the logical approach: 

  • The opinion will be in the form of a transposed statement about the propositions, 
  • The opinion will not be logically sound or coherent, 
  • There will be no way for the trier to properly dis-entangle the expert evidence to apply it in their own reasoning, and 
  • There would be a legitimate concern that the expert’s opinion might usurp that of the Court to some degree.  

Through the adoption, and proper application, of the logical approach these issues can all be avoided.

In my experience lawyers tend to be good at reasoning, but in this particular argument they demonstrate a complete and utter misunderstanding of the logical approach, how it works, and what information it provides. They understand what is required of the expert, yet fail to appreciate two critical things:

  • The traditional approach fails to achieve what is required by the court, and
  • The logical approach is the only way to actually achieve that objective.  

Sadly, little will change until this becomes the common understanding amongst the various parties involved in our Justice system. I fear that may be a long way off.  


  1. The logical approach to evidence evaluation has also been called ‘evaluative reporting’ in some articles. Different name, but it’s the same thing.
  2. This is more of a concern with a jury, rather than a judge. But it exists in all such situations.
  3. Of course, these concepts are neither new or recent. The logical approach has been known (and used by some examiners) since the late 1800’s.
  4. Please understand I am not saying this is how a court should function. It makes a lot of sense but, in the end, how things are done is entirely up to the judge.

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