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'Heads I win, tails you lose': testing directional alternative hypotheses in ecological and evolutionary research
1Dept. of Biology, University of California, Santa Cruz, CA 95064, USA.
Abstract:
Whenever experiments make a priori predictions about the direction of change in some parameter, one-tailed test statistics offer a potentially large gain in power over the corresponding two-tailed test. This gain is rarely used in ecology and evolution because of (1) the belief that one-tailed procedures are unavailable for most statistical tests and (2) an inherent dilemma in one-tailed tests: how do we handle large parameter changes in the unanticipated direction? The first problem is a misconception, whereas the second is easily resolved by recognizing that one- and two-tailed tests are simply extremes in a continuum of testing options.
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