Hi Graham,

The help from 3dcalc seems to give some details, along with a nice
example.

---- from '3dcalc -help':
orstat(n,a,b,c,...) computes the n-th order statistic of
{a,b,c,...} - that is, the n-th value in size, starting
at the bottom (e.g., orstat(1,a,b,c) is the minimum)
----

So in saying "the n-th value in size, starting at the bottom", it
implies returning a value (not an index) from the sorted list. The
example of orstat(1,a,b,c) being the minimum should be very useful.
So orstat(3,a,b,c) is the maximum, and orstat(9,a,b,c,d,e,f,g,h,i,j)
is the second largest of the 10 values.

I hope that helps,

- rick