`My question is a conceptual question regarding how to interpret a two-
``sided permutation test under Stata 11. [My question is framed around
``Example 1 of the permute command in the Stata Reference Manual for
``Stata 11 for those who has the documentation handy (page 1312).]
`
Under the permute command:

`1. If the test statistic is the sum of the observations of the
``treated group, and
``2. if all the values for both the treated and the untreated group are
``positive,
`

`then there is no difference in the reported p-value between a one-
``sided test versus a two-sided test.
`

`This seems strange! After thinking about this puzzling result, I
``realized that this result is driven by the fact that the values of the
``sampling distribution are positive because the values for both the
``treated and the untreated groups are all positive. And given how
``Stata defines the two-sided test for permute, there is no difference
``between the one-sided and the two-sided test in this case.
`

`I can understand the mechanics of how Stata calculated the p-value for
``a one-sided and a two-sided test, but the definition of the two-sided
``test in this context seems counter-intuitive. For example, a
``parametric equivalent situation is a two-sided Chi-square test because
``the Chi-square distribution only has positive values. But in
``constructing the two-sided test for the Chi-square distribution, we
``don't ignore the left-tail of the two-sided test by defining the two-
``sided test with absolute values as we do in defining the two-sided
``test under the permutation test.
`
-- John Lin
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