Algorithm for computing Excel PercentRank
 

I am current trying to determine how Excel calculates PercentRank.

The project I am working on requires the use of this function – and we
cannot use COM automation to call Excel.

The only documentation I could find was the following:

Excel Function: PERCENTRANK(x, array)

- If x matches one of the values in the array, this function is
equivalent to the Excel formula = (RANK(x)-1)/(N-1) where N is the
number of data points.

- If x does not match one of the values, then the PERCENTRANK function
interpolates.

Rank(x) is easy to compute. The second statement is where I'm having
trouble with, how does Excel "interpolates" the percentage rank? What
method does it use?

Thanks in advance,
Kam Mistry
Microsoft .NET Developer (C#)