Github user abalanonline commented on the issue:
https://github.com/apache/jmeter/pull/296
Hello @pmouawad ,
Different estimation types used by Apache Math explained
[here](https://en.wikipedia.org/wiki/Quantile#Estimating_quantiles_from_a_sample).
The use case for nearest rank method is simple. If the 90% percentile is
1200 ms than that means that 90% of tests take no more than 1200 ms. Same for
95% and 99%. Very informative value for JMeter. It is implemented in R_1 and
R_2.
The purpose of interpolated method is to eliminate the discontinuity and to
get data useful for calculation in different linear functions. Difference is in
the min and max values. If we treat them only as starting and ending points in
the diagram then C=1 variant (R_7) is used. Otherwise if the min and max are
the full value experiment results and deserve their own space on a diagram then
C=0 variant (LEGACY, R_6) is considered.
Intermediate variant C=1/2 (R_5, R_8, R_9) is fine too because it is
balanced and does not distort the "angle" of a diagram.
The bug affects most of the libraries because of a cognitive inertia of
developers. The ceiling function is very rarely used and it is psychologically
easier to use round instead. Also round function give correct median for odd
number of experiments. Seeing that they calm down and believe that everything
is calculating correctly.
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