Fractions of observing time for which the strongest periods are detectable as a function of detection threshold. The solid lines are the average fractions of time derived from all available observations, while the dotted lines (with labels in italics) are the fractions of time derived from the single observation in which the period is seen most frequently. For this plot, power spectra from directly adjacent, non-overlapping time intervals have been used. For no system, the period was detected 100% of the time, showing the transient nature. Furthermore, the signal is episodic because the fractions are not the same in all observations.