An apparatus and method are provided for detecting regularity in a number of occurrences of an event observed during multiple instances of a counting period. The apparatus has regularity detection circuitry for seeking to detect such a regularity, and a storage providing a storage entry having a count value field to store a count value and a confidence indication field to indicate a confidence in the regularity. The regularity detection circuitry is arranged to consider the multiple instances of the counting period in pairs, for one instance in a given pair of the pairs the regularity detection circuitry incrementing the count value following each occurrence of the event, and for the other instance in the given pair the regularity detection circuitry decrementing the count value following each occurrence of the event. Check circuitry is then arranged, following completion of both counting periods in the given pair, to adjust the confidence indication to indicate an increased confidence when it is determined that the count value has returned to an initial value, and otherwise to adjust the confidence indication to indicate a decreased confidence and to reset the count value to the initial value. Such an approach provides a particularly storage efficient mechanism for seeking to detect regularity in a number of occurrences of an event.