Appendix B: Understanding the Statistics

Statistical Terms

The following table lists general terms used for the statistics.

Term	Description
Percentile	A percentile is the value of a variable below which a certain percent of observations fall. Thus the 30th percentile is the value (or score) below which 30 percent of the observations may be found.
Average	The arithmetic mean. For timers, average is the total amount of time counted by the timer (not the elapsed time) divided by the Count (that is, the total number of readings). For example, the average for Transaction Time is the amount of time it took to complete all the successful transactions divided by the number of successful transactions (the Count).
Standard deviation (stddev)	The standard deviation is a simple measure of the variability or dispersion of a data set. A low standard deviation indicates that all of the data points are very close to the same value (the mean). A high standard deviation indicates that the data is "spread out" over a large range of values.