
A command that becomes available in the Query submenu when you select a PointProcess object.
This command will write into the Info window the local jitter, which is the average absolute difference between consecutive intervals, 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 local jitter can be used as a measure of voice quality; it is the most common jitter measurement and is usually expressed as a percentage. See Voice 2. Jitter.
(In the following the term absolute means two different things: (1) the absolute (i.e. nonnegative) value of a real number, and (2) the opposite of relative.)
The local jitter is defined as the relative mean absolute secondorder difference of the point process (= the firstorder difference of the interval process), as follows.
First, we define the absolute (nonrelative) local jitter (in seconds) as the mean absolute (nonnegative) difference of consecutive intervals:
jitter(seconds) = ∑_{i=2}^{N} T_{i}  T_{i1} / (N  1) 
where T_{i} is the duration of the ith interval and N is the number of intervals. If an interval T_{i1} or 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, the term T_{i}  T_{i1} is not counted in the sum, and N is lowered by 1 (if N ends up being less than 2, 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 local jitter.
Finally, we compute the (relative) local jitter as
jitter = jitter(seconds) / meanPeriod(seconds) 
The result is a value between 0 and 2, or between 0 and 200 percent.
© ppgb, March 2, 2011