[tab]If the contingency table is 2 by 2, Fisher's Exact Test for
[tab]independence is computed. Note that this test is based on the
[tab]assumption that the row and column totals are fixed, which may or
[tab]may not be appropriate depending on how the data were generated.
[tab]The left p-value should be used when the alternative to
[tab]independence is negative association (values tend to cluster
[tab]in the lower left and upper right cells); the right p-value should
[tab]be used if the alternative is positive association. The two-tailed
[tab]p-value for this test is calculated by method (b) in Agresti
[tab](1992, section 2.1): it is the sum of the probabilities of all
[tab]possible tables having the given row and column totals and having
[tab]a probability less than or equal to that of the observed table.
