On 10 Jan 2009 at 16:00, Jim Clark wrote: > Would not a fairer characterization of one-tailed tests be that the > unanticipated outcome would be a statistical artefact, rather than > calling it "totally meaningless?" Would not that apply to a whole > host of "possible" (but theoretically/empirically unexpected) > outcomes, including:
> - better memory for abstract words than concrete words > - people who are dissimilar to one another liking each other better than > those who are similar > - faster reading times for long, unfamiliar, irregular words than short, > familiar, regular words > - older children doing worse at an arithmetic task than younger children > - children with ADHD doing better on an attention task or the stop-signal RT > task > In essence, does not every well-established effect constitute grounds > for viewing a contradictory outcome as an artefact, and hence justify > a one-tailed test? I think I may not grasp Jim's argument here, or perhaps he doesn't follow mine. Because, in the examples Jim gives, I'd have to say that every one provides an important meaningful result even if unexpected, and therefore only two-tailed tests will do. By restricting one-tailed tests to only those cases where a result in the wrong tail is "meaningless", I'm saying that such an outcome would be considered of no practical or theoretical value, and so one would never be tempted to make use of such a finding. As I've said, such examples are hard to find. A new therapy or drug which does worse than placebo is my best (but still imperfect) example. By diligent surfing, I've come up with another. Suppose you're checking quality control on an assembly line for widgets, and want to know when quality falls below a set value. You couldn't care less if quality instead improves; you only intend to take steps if it deteriorates. A one- tailed test would do here. But other than such contrived and certainly rare cases, only two-tailed tests will do. Any standard deviate who uses one-tailed tests to massage his data should be gonged with a bell curve. More surfing took me to a generous helping at Google books of Abelson's (1995) _Statistics as Principled Argument_, where he considers the issue. You can get there by using the search terms in ordinary Google of "one- tailed" and "meaningless"; Abelson is the first entry. Then scan backwards to the start of the chapter, "Styles of Rhetoric" (p. 54). In Abelson's terms, I'd call myself a "conservative" on this issue (but not, I protest, in any other way). Stephen ----------------------------------------------------------------- Stephen L. Black, Ph.D. Professor of Psychology, Emeritus Bishop's University e-mail: [email protected] 2600 College St. Sherbrooke QC J1M 1Z7 Canada Subscribe to discussion list (TIPS) for the teaching of psychology at http://flightline.highline.edu/sfrantz/tips/ ----------------------------------------------------------------------- --- To make changes to your subscription contact: Bill Southerly ([email protected])
