True – it is not a particularly convincing way of putting the argument, but the point is nevertheless an important one.
There are two principal reasons why the DTR test is not, without more, an appropriate device for calculating probability of causation. The first is that the relative ‘risk’ on which it is based is not in any technical sense a risk at all,
but merely a measure of incidence over a defined population. To treat it as substitutive for probability, therefore, is to confuse two distinct concepts, with potentially adverse consequences for the causal inquiry.
The second reason is that the result reached by the relative risk calculation involved gives a ‘net result’. That is, it tells us how many
extra cases of a disease are caused by the factor being tested. It does not, therefore, include those cases in which the adverse outcome would have happened anyway, just not when it did. In other words, it only identifies cases which would
never otherwise have happened, and does not account for those which the factor merely accelerated. So, the DTR test and the measures on which it relies can
underestimate a factor’s effect on a population. What it cannot do, however, is overestimate that effect.
The conclusion, therefore, that the incidence of injury has been doubled by the factor being tested amounts to a single indication that there might exist a statistical association between that factor and the injury. This is not, however,
the same thing as establishing on the balance of probabilities that the factor caused the injury: such a statistic is just one component of the range of evidence on which a court should reach its decision, since it tells us nothing about what happened
in any particular case.
Best,
Sarah
The argument that the doubling-the-risk test should be rejected because "a doubled tiny risk is still very small" is not especially convincing. It is true that if the defendant's
actions increase the risk of the claimant suffering harm H from one in a million to three in a million that may not seem like a big deal. And it may be a reason for concluding that the defendant has not fallen below the standard of care. But if that the
defendant has fallen below the standard of care and if the harm (the three in a million chance) has occurred, then why not conclude that, on a balance of probabilities, the defendant caused it?
Dear colleagues,
In Williams v Bermuda Hospitals Board at
https://www.jcpc.uk/cases/docs/jcpc-2014-0110-judgment.pdf, the PC grapples with the thorny issue of material contribution to injury.
The acknowledgement that the Court of Appeal in
Bailey v MOD was wrong to describe material contribution to injury as an exception to the But For test is welcome, as is the recommendation that the doubling of the risk test should only ever be used with caution.
The dismissal of the Bermuda Hospital Board’s appeal , however, on facts which did not require a material contribution to injury analysis is not good news, particularly for medical practitioners and their
insurers. An orthodox But For analysis along Barnett lines should have been applied, under which a different result would have been reached – the evidence (patchy thought it was) suggested that the claimant’s fate was sealed before the defendant contributed
to the ongoing risk.
Best wishes,
Sarah
Sarah Green
Lord Hoffmann Fellow in Law
St Hilda's College
Oxford
OX4 1DY
sarah.green@law.ox.ac.uk
01865 286661
https://www.law.ox.ac.uk/people/sarah-green