There is no universally accepted definition of a percentile.
Using the 65th percentile as an example, some statisticians define the 65th
percentile as the lowest score that is larger than 65% of the scores. Others
have defined the 65th percentile as the smallest score that is greater than
or equal to 65% of the scores. A more sophisticated definition is given below.
The first step is to compute the rank (R) of the percentile in question. This is done using the following formula:
R = P/100 x (N + 1)
where P is the desired percentile and N is the number of numbers. If R is an integer, then the Pth perentile is the number with rank R. When R is not an integer, we compute the Pth perentile by interpolation as follows: