There is not a straightforward way to do a Student t-test in Ferret,
but such a script could be written, I think. It requires evaluating
the Gamma function, which would be tricky in Ferret because it is an
integral that is evaluated until it converges. I've attached the
fortran code I use (from Numerical Recipes) in case someone smarter
than me wants to write it into a Ferret script (and please post to the
list!).
The Wikipedia article (Student_t_distribution) is pretty good.
One very important thing to note when doing a Student t-test is that
you must know the degrees of freedom first! That is a separate
calculation that is typically found as an integral over the product of
the autocorrelation functions of the two variables. There are several
other essential subtleties, that are described in, among other places,
p3015 of Kessler et al (1996, JPO, link to the pdf at the end of this
message).
Statistics are hard.
Billy K
Link to the paper with description of the procedure and assumptions
(pdf, see p3015):
http://faculty.washington.edu/kessler/abstracts/ksmh-1996.pdf