Queues are erratic, building up suddenly with no warning,  then reducing rapidly or persisting for some time.  Waiting  time is the most important aspect of a queue, but there is no completely satisfactory measurement  of waiting time.

First chart shows the familiar bell-shape  followed by many natural statistics like height and  weight.  The average is the most common value. Values cluster around the average, which is a good representation of what is being measured.

Waiting times in call-centres have a different pattern from the bell curve. The most common value is zero, seen by a majority of callers.  Average delay (wait time for calls not answered immediately) can be several times the overall average. The average is a poor representation of wait times.

Instead of average wait, a commonly used measurement is "service level", which is the % of customers who wait less than some  target time. Call centres typically have a target like answering 80% of calls within 15 seconds.