<P> Advice concerning the use of one - tailed hypotheses has been inconsistent and accepted practice varies among fields . The greatest objection to one - tailed hypotheses is their potential subjectivity . A non-significant result can sometimes be converted to a significant result by the use of a one - tailed hypothesis (as the fair coin test, at the whim of the analyst). The flip side of the argument: One - sided tests are less likely to ignore a real effect . One - tailed tests can suppress the publication of data that differs in sign from predictions . Objectivity was a goal of the developers of statistical tests . </P> <P> It is a common practice to use a one - tailed hypotheses by default . However, "If you do not have a specific direction firmly in mind in advance, use a two - sided alternative . Moreover, some users of statistics argue that we should always work with the two - sided alternative ." </P> <P> One alternative to this advice is to use three - outcome tests . It eliminates the issues surrounding directionality of hypotheses by testing twice, once in each direction and combining the results to produce three possible outcomes . Variations on this approach have a history, being suggested perhaps 10 times since 1950 . </P> <P> Disagreements over one - tailed tests flow from the philosophy of science . While Fisher was willing to ignore the unlikely case of the Lady guessing all cups of tea incorrectly (which may have been appropriate for the circumstances), medicine believes that a proposed treatment that kills patients is significant in every sense and should be reported and perhaps explained . Poor statistical reporting practices have contributed to disagreements over one - tailed tests . Statistical significance resulting from two - tailed tests is insensitive to the sign of the relationship; Reporting significance alone is inadequate . "The treatment has an effect" is the uninformative result of a two - tailed test . "The treatment has a beneficial effect" is the more informative result of a one - tailed test . "The treatment has an effect, reducing the average length of hospitalization by 1.5 days" is the most informative report, combining a two - tailed significance test result with a numeric estimate of the relationship between treatment and effect . Explicitly reporting a numeric result eliminates a philosophical advantage of a one - tailed test . An underlying issue is the appropriate form of an experimental science without numeric predictive theories: A model of numeric results is more informative than a model of effect signs (positive, negative or unknown) which is more informative than a model of simple significance (non-zero or unknown); in the absence of numeric theory signs may suffice . </P>

If the null hypothesis in a hypothesis test includes the term = then the test is a