  Emergence of noise induced regularity or Coherence Resonance in nonlinear
excitable systems is well known. We explain theoretically why the normalized
variance ($V_{N}$) of inter spike time intervals, which is a measure of
regularity in such systems, has a unimodal profile. Our semi-analytic treatment
of the associated spiking process produces a general yet simple formula for
$V_{N}$, which we show is in very good agreement with numerics in two test
cases, namely the FitzHugh-Nagumo model and the Chemical Oscillator model.
