4 How non-experts can scrutinise experts
A fundamental challenge with the use of expert evidence is an apparent conflict between the roles of the court and the expert. On the one hand, the court is supposed to be the decision maker but does not possess relevant expertise on crucial issues. On the other hand, the expert or experts possess the relevant expertise but are only supposed to assist the court – they are not permitted to engage in decision making. This seems to put the court in a difficult position where there is a single expert or disagreement between experts; how could a non-expert decision maker conclude that the opinion of an expert should not be followed?
This apparent contradiction was expressed by the US Judge Learned Hand as follows:
The whole object of the expert is to tell the jury, not facts … but general truths derived from his specialized experience. But how can the jury judge between two statements each founded upon an experience confessedly foreign in kind to their own? It is just because they are incompetent for such a task that the expert is necessary at all … When the conflict is direct and open, the absurdity of our present system is apparent.
Advocates appear similarly nervous about cross-examining experts, and the Law Commission (2011) has suggested that they tend to focus on undermining the credibility of the expert rather than challenging the substance of their opinions. Yet judges and juries routinely undertake this task, for better or worse, so it is worth examining how they might achieve this. To illustrate how this may be done, read the passage from a Sherlock Holmes story in Box 3.
Box 3 The adventure of the dancing men
Holmes had been seated for some hours in silence with his long, thin back curved over a chemical vessel in which he was brewing a particularly malodorous product. His head was sunk upon his breast, and he looked from my point of view like a strange, lank bird, with dull grey plumage and a black top-knot.
‘So, Watson,’ said he, suddenly, ‘you do not propose to invest in South African securities?’
I gave a start of astonishment. Accustomed as I was to Holmes’s curious faculties, this sudden intrusion into my most intimate thoughts was utterly inexplicable.
‘How on earth do you know that?’ I asked.
He wheeled round upon his stool, with a steaming test-tube in his hand and a gleam of amusement in his deep-set eyes.
‘Now, Watson, confess yourself utterly taken aback,’ said he.
‘I am.’
‘I ought to make you sign a paper to that effect.’
‘Why?’
‘Because in five minutes you will say that it is all so absurdly simple.’
‘I am sure that I shall say nothing of the kind.’
‘You see, my dear Watson’ – he propped his test-tube in the rack and began to lecture with the air of a professor addressing his class – ‘it is not really difficult to construct a series of inferences, each dependent upon its predecessor and each simple in itself. If, after doing so, one simply knocks out all the central inferences and presents one’s audience with the starting-point and the conclusion, one may produce a startling, though possibly a meretricious, effect. Now, it was not really difficult, by an inspection of the groove between your left forefinger and thumb, to feel sure that you did not propose to invest your small capital in the goldfields.’
‘I see no connection.’
‘Very likely not; but I can quickly show you a close connection. Here are the missing links of the very simple chain: 1. You had chalk between your left finger and thumb when you returned from the club last night. 2. You put chalk there when you play billiards to steady the cue. 3. You never play billiards except with Thurston. 4. You told me four weeks ago that Thurston had an option on some South African property which would expire in a month, and which he desired you to share with him. 5. Your cheque-book is locked in my drawer, and you have not asked for the key. 6. You do not propose to invest your money in this manner.’
‘How absurdly simple!’ I cried.
‘Quite so!’ said he, a little nettled. ‘Every problem becomes very childish when once it is explained to you.’
Sherlock Holmes is smart, but not necessarily an expert. Nonetheless, the story illustrates how another person (Watson), who is initially baffled by the inference drawn by Holmes, can be satisfied that his opinion is robust when Holmes spells out his thinking. Watson does not need to be able to make the inference himself; he only needs to be able to follow the individual links in the chain of reasoning one at a time to satisfy himself that each link is reasonable (or unreasonable). This is the type of task that a legal decision maker may be capable of.
Logicians divide this checking process into two different types:
Checking for truth is where the decision maker checks whether the factual assumptions relied upon by the expert are true. For example, if the expert says that the brakes on the car were old and worn, the judge or juror can check the evidence to see if that is correct. As the court said in R v Turner,Footnote 22 ‘[b]efore a court can assess the value of an opinion it must know the facts upon which it is based. If the expert has been misinformed about the facts or has taken irrelevant facts into consideration or has omitted to consider relevant ones, the opinion is likely to be valueless.’
- Checking for validity is where the decision maker checks that the logical inferences made by the expert are correct. For example, if the expert multiplies two probabilities, the judge or juror can also do the multiplication themselves to check that the answer is right. In Bolitho v City and Hackney Health Authority,Footnote 23 the House of Lords said that if expert evidence is not capable of withstanding logical analysis, then the court is entitled to reject it. An illustration of a court using a validity error to overturn a conviction is that of R v T,Footnote 24 which we will look at in a bit more detail towards the end of the course. In that case, footprints from the crime scene showed that the culprit’s trainers had damage that did not match the trainers later recovered from the accused. However, the expert assigned this evidence a ‘likelihood ratio’ of 1 (meaning that the evidence was neither exculpatory nor inculpatory). Logically, the value should have been less than 1 (because it was exculpatory). The court was rightly sceptical of the expert’s opinion and upheld the appeal.
All things being equal, if the links in a chain of expert reasoning are true and valid, that is a reason to agree with the expert. But if a link is false or invalid, that is a reason to disagree with the expert. This explains the obligation on an expert to give reasons for their opinion that include the facts and assumptions on which it is based.
Footnotes
- 22 R v Turner [1975] QB 834, 840.Back to main text
- 23 Bolitho v City and Hackney Health Authority [1998] AC 232.Back to main text
- 24 R v T [2010] EWCA Crim 2439.Back to main text