  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, edge-localised plasma instabilities (ELMs) in
magnetically confined fusion (MCF) experiments. The development of such
techniques is essential to ensure tolerable heat loads on components in large
MCF fusion devices, and is necessary for their development into economically
successful power plants. 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.
Example applications include assessing whether triggering of ELMs is a
statistical or deterministic process, and the establishment of thresholds to
ensure that ELMs are reliably triggered.
