Subjective probability and causality assessment

Serious adverse reactions to a drug usually occur too rarely to be identified in the clinical trials required to demonstrate efficacy before the drug can be put on the market. Instead, they are generally first encountered in the uncontrolled world of everyday clinical practice, and the industrial and national regulatory agencies that are responsible for drug safety must rely for their first indications of a possible drug‐adverse event connection on case reports submitted to them by practitioners who observe an occurrence of the event in one of their patients taking the drug. A typical situation in the work of these agencies finds a group of experts assembled around a table reviewing a small series of case reports (perhaps as small as one!) that link a particular drug with a particular type of adverse event. The experts want to determine whether, for each case in their series, the available evidence indicates that the drug caused the adverse event to occur. How are they to proceed? This paper outlines an approach to this problem, based on the use of subjective probability.