Ranks and interpolation

Quantiles divide an ordered distribution into parts: Q1 is the 25th percentile, Q2 the median and Q3 the 75th percentile. Decile Dj is percentile 10j. This pack uses rank r=(n+1)p for ungrouped interpolation when r is within the available ranks. If r=j+d, use (1-d)x(j)+d x(j+1). Here d is the fractional part of the rank, not necessarily p. Other conventions are valid when explicitly specified.