
A command that becomes available in the Query submenu when you select a PointProcess object.
This command will write into the Info window the fivepoint Period Perturbation Quotient, a jitter measure defined as the average absolute difference between an interval and the average of it and its four closest neighbours, divided by the average interval (an interval is the time between two consecutive points).
As jitter is often used as a measure of voice quality (see Voice 2. Jitter), the intervals are often considered to be glottal periods. For this reason, the command has settings that can limit the possible duration of the interval (or period) or the possible difference in the durations of consecutive intervals (periods).
The jitter can be used as a measure of voice quality. See Voice 2. Jitter.
The fivepoint Period Perturbation Quotient (PPQ5) is defined in terms of five consecutive intervals, as follows.
First, we define the absolute (i.e. nonrelative) PPQ5 (in seconds):
absPPQ5(seconds) = ∑_{i=3}^{N2} T_{i}  (T_{i2} + T_{i1} + T_{i} + T_{i+1} + T_{i+2}) / 5 / (N  4) 
where T_{i} is the duration of the ith interval and N is the number of intervals. If an interval T_{i2} or T_{i1} or T_{i} or T_{i+1} or T_{i+2} is not between Period floor and Period ceiling, or if T_{i2}/T_{i1} or T_{i1}/T_{i2} or T_{i1}/T_{i} or T_{i}/T_{i1} or T_{i+1}/T_{i} or T_{i}/T_{i+1} or T_{i+2}/T_{i+1} or T_{i+1}/T_{i+2} is greater than Maximum period factor, the term T_{i}  (T_{i2} + T_{i1} + T_{i} + T_{i+1} + T_{i+2}) / 5 is not counted in the sum, and N is lowered by 1 (if N ends up being less than 5, the result of the command is undefined).
Second, we define the mean period as
meanPeriod(seconds) = ∑_{i=1}^{N} T_{i} / N 
where T_{i} is the duration of the ith interval and N is the number of intervals. If an interval T_{i} is not between Period floor and Period ceiling, or if T_{i1}/T_{i} or T_{i}/T_{i1} is greater than Maximum period factor and T_{i+1}/T_{i} or T_{i}/T_{i+1} is greater than Maximum period factor, the term T_{i} is not counted in the sum, and N is lowered by 1; this procedure ensures that in the computation of the mean period we use at least all the intervals that had taken part in the computation of the absolute PPQ5.
Finally, we compute the fivepoint Period Perturbation Quotient as
PPQ5 = PPQ5(seconds) / meanPeriod(seconds) 
The result is a value between 0 and 4, or between 0 and 400 percent.
© ppgb, March 2, 2011