The generic question is considered: How can we determine the probability of an otherwise quasirandom event, having been triggered by an external influence? A specific problem is the quantification of the success of techniques to trigger, and hence control, plasma instabilities in magnetically confined fusion (MCF) experiments. The successful development of such techniques is essential to ensure tolerable heat loads on components in large fusion devices, something that is necessary for the development of an economically successful MCF power plant. Bayesian probability theory is used to rigorously formulate the problem and to provide a formal solution. Accurate but pragmatic methods are developed to estimate triggering probabilities, and are illustrated with experimental data. These allow results from experiments to be quantitatively assessed, and rigorously quantified conclusions to be formed.