(defn test-statistic-distribution
"
	Create a distribution of the test-statistic over the possible
	random samples of treatment units from the possible units.

	There are two methods for generating the distribution. The
	first method is enumerating all possible randomizations and
	performing the test statistic on each. This gives the exact
	distribution, but is only feasible for small problems.

	The second method uses a combination-distribution to sample
	for the space of possible treatment assignments and applies
	the test statistic the sampled randomizations. While the
	resulting distribution is not exact, it is tractable for
	larger problems.

	The algorithm automatically chooses between the two methods
	by computing the number of possible randomizations and
	comparing it to *test-statistic-iterations*. If the exact
	distribution requires fewer than *test-statistic-iterations*
	the enumeration method is used. Otherwise, it draws
	*test-statistic-iterations* total samples for the simulated
	method.

	By default, the algorithm uses parallel computation. This is
	controlled by the function *test-statistic-map*, which is
	bound to pmap by default. Bind it to map to use a single
	thread for computation.

	Arguments:
		test-statistic	A function that takes two vectors and summarizes
				the difference between them
		n		The number of total units in the pool
		k	  The number of treatment units per sample

	See also:
		combination-distribution, pdf, cdf, draw, support

	References:
		http://en.wikipedia.org/wiki/Sampling_distribution
		http://en.wikipedia.org/wiki/Exact_test
		http://en.wikipedia.org/wiki/Randomization_test
		http://en.wikipedia.org/wiki/Lady_tasting_tea

	Examples:
		
"
	[test-statistic n k]
	; for now returns entire set of computed values, should summarize via frequencies
  (*test-statistic-map* test-statistic ; *t-s-m* is bound to pmap by default
    (let [cd (combination-distribution n k)]
  		(if (> (nCk n k) *test-statistic-iterations*)
      ; simulated method
        (repeatedly *test-statistic-iterations* #(draw cd))
      ; exact method
      	(combinations (range 0 n) k)))))