Description:
Provides more details on the percentile calculations in APM CE (CEM) Analysis Graph Statistics Reports. Of particular interest is the calculation for the 95th percentile.
For example, the transaction time (seconds) for a set of 10 transactions is: 1, 1, 2, 2, 2, 2, 2, 2, 2, 10, 10. (Note that these transactions are sorted from fastest to slowest.)
What does APM CE (CEM) show for the 95 percentile time? Will it be 10 seconds or a value doing some averaging?
Solution:
In the Analysis Graph data for Time, the n^{th} percentile means the n percent of the transactions having a response time at that level or less.
APM CE(CEM) uses standardized statistical percentiles.. i.e. Percentile values are based on ranking the actual values. So, the percentile value will be an actual value/data point and not an averaged value. (This will be along similar lines to the median calculation equaling 50% percentile versus average).
Review this wiki page for some good explanation and examples of "Nearest rank" for percentiles. (Note: This is a term that is part of. standard statistical terminology: http://en.wikipedia.org/wiki/Percentile
So, for the scenario of 10 transaction time values:
2 x 1 second
6 x 2 seconds
2 x 10 seconds
The 75% percentile will be the value that ranks as (75/100)*10 = 7.5 = 8th highest value i.e. 2 seconds.
The 95% percentile will be the value that ranks as (95/100)*10 = 9.5 = 10th highest value i.e. 10 seconds.
Additional information can be found in the CA APM Overview Guide and the "Analysis Graphs - Time" online help. There are more details in the Glossary section on related terms::
average value, box-whisper graph, lower specification limit, median value, percentile value, range, span, upper specification limit.