Relative Average Perturbation (RAP) is a quantitative measure of the voice on a sustained vowel. It one of the many ways of measuring the frequency perturbation which minimises the effects of slow changes in the fundamental frequency (F0  or Fx) by averaging across three successive vibratory cycles provide an estimate of the value of the middle period had there been no perturbation present (known as a “corrected” period value). The jitter for a given cycle is then taken to be the difference between the corrected period and the actual period. It is thought to provide a more reliable measure of the intrinsic stability of the voice than other jitter measures.