Re: One or Two tails: II ?

*** Marvin Zelen dismisses one-sided tests in another way--he finds them unethical! His argument is as simple as it is elegant. Put in >terms of comparing a new treatment to standard, anyone who insists on a one- tailed test is saying the new treatment can't do worse >than the standard. If the new treament has any effect, it can only do better. The method of analysis should be part of a study's design. >If investigators are prepared to justify the use of a one-tailed test at the start of the study, then it is unethical not to give the new >treatment to everyone!

I agree with Zelen's argument in that context, but there are other
limited contexts which make some directional tests sensible and
ethical. For example in a replication and extension one might introduce
a manipulation/intervention expected to influence effect A but not
effect B. If effect B is well established in the literature one might
use the more powerful one sided test to establish that your
participants demonstrated effect B. For effect A one would use a two
sided test.

Thom

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