st: one-tailed tests

["Eric Uslaner" <euslaner@gvpt.umd.edu>]

Bea Potter asked how to do one-tailed tests and Roger Newson responded: "I don't know why anybody would want to do a 1-tailed test, except if the 
distribution of the test statistic, under the null hypothesis, really IS one-tailed." 

I don't understand why anyone would do a two-tailed test.  If you have a theory, that theory is directional: The more A, the more B.  A two-tailed test is completely inappropriate for this.  E.g., in studies of voting behavior in political science, we usually posit that party identification is the dominant force behind voter choice.  So if you are a strong Democrat, you will vote Democratic.  A two-tailed test would imply that a strong Republican would be equally likely as a strong Democrat to vote Democratic.  This is plain silly.  If you don't have a theoretical focus, you don't have a research design worth submitting anywhere.  

So I don't understand why anyone does two-tailed tests and why Stata (together with other statistical programs) report them as defaults.

Ric Uslaner




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