compute rank of samples, the mean value is used in case of ties
rank of vector X
this function is just a wrapper for RANKS, and provided for compatibility with the statistics toolbox of matlab(tm)
//Counting from smallest to largest, the two 20 values are 2nd and 3rd, so they both get rank 2.5 (average of 2 and 3): nan_tiedrank([10 20 30 40 20]);