  Period variability, quantified by the standard deviation (SD) of the
cycle-to-cycle period, is investigated for noisy phase oscillators. We define
the checkpoint phase as the beginning/end point of one oscillation cycle and
derive an expression for the SD as a function of this phase. We find that the
SD is dependent on the checkpoint phase only when oscillators are coupled. The
applicability of our theory is verified using a realistic model. Our work
clarifies the relationship between period variability and synchronization from
which valuable information regarding coupling can be inferred.
