The cycle-to-cycle variability of the period amplitude of vocal cord vibration. Shimmer is used as one of the measures for the micro-instability of vocal cord vibrations. Given that shimmer may vary dramatically with changes of voice production within the same voice, and also that a given shimmer measurement is non-specific to any particular voice disorder, this measure is considered superfluous to the integrative diagnostic model. Shimmer is therefore not used routinely at our practice.