The balance of probabilities, medical risk and Bayesian prior probability

For a lawyer involved in civil claims, the balance of probabilities is the “bread and butter” of litigation and is the standard of proof.  For a scientist, and therefore for someone medically trained, probability is something entirely different and something much more nuanced.  A recent example of the different use of words of probability in law and medicine was demonstrated in Rich v Hull and East Yorkshire Hospitals NHS Trust [2015] EWHC 3395.

The Scientific Advisory Committee of the Royal Society of Gynaecologists published in 1992 “Antenatal Corticosteroid Administration Reduces the Incidence of Neonatal RDS”.  This encouraged the use of Betamethasone or Dexamethasone therapy “when delivery is likely before 24 weeks”.  Evidence given to the judge about this guidance indicated that “likely” was a synonym for “probable”, and that this was intended to convey the idea of “significant degree of likelihood”.  Further it addressed risk rather than issues of proof.  One eminent witness indicated his view that a risk of between 5 – 20% would be sufficient to trigger the therapy, possibly adjusted upwards to reflect divergence of view amongst colleagues.  The 5-20%, he explained, reflected a Bayesian prior probability, the generic probability, before considering the circumstances of the individual case.

The judge concluded from the evidence that “likely” in the SAC guidance did not mean “probably” in legal terms but something more akin to “a real risk”.  However he also recognised that “with this type and level of analysis … it is artificial to reduce clinical judgments to the sort of exact formulation to which lawyers aspire”.  In the end the duty of care the judge identified was that the doctor should have considered prescribing maternal corticosteroids if he had, or ought to have had, a clinical suspicion that the mother might or would deliver within the relevant period.

The importance of medical witnesses in legal proceedings understanding the difference of approach to probability and likelihood, and to the terms used, of the two disciplines (law and medicine) is well illustrated by this case.  The danger of misunderstandings arising on points of considerable importance in litigation is apparent.  For those who have read ‘Writing Medico-Legal Reports in Civil Claims – an essential guide’, this is of course another example of a case of “foot-wearing”.

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