HEADS I WIN, TAILS YOU LOSE - TESTING DIRECTIONAL ALTERNATIVE HYPOTHESES IN ECOLOGICAL AND EVOLUTIONARY RESEARCH

Whenever experiments make a priori predictions about the direction of change in some parameter, one-tailed test statistics offer a potential ly large gain in power over the corresponding two-tailed test. This ga in is rarely used in ecology and evolution because of (1) the belief t hat one-tailed procedures are unavailable for most statistical tests a nd (2) an inherent dilemma in one-tailed tests: how do we handle large parameter changes in the unanticipated direction? The first problem i s a misconception, whereas the second is easily resolved by recognizin g that one and two-tailed tests are simply extremes in a continuum of testing options.