One difference between the two numbers is in the way outliers affect the calculation of the
average: since they are included as part of the average, large outliers will have a large effect
on the average response time.
Figure 2-2 shows a graph of 20 requests with a somewhat typical range of response times.
The response times range from 1 to 5 seconds. The average response time (represented by
the lower heavy line along the x-axis) is 2.35 seconds, and 90% of the responses occur in 4
seconds or less (represented by the upper heavy line along the x-axis).
Figure 2-2. Typical set of response times
This is the usual scenario for a well-behaved test. Outliers can skew that analysis, as the data
in Figure 2-3 shows.