ERIC Number: ED364598
Record Type: RIE
Publication Date: 1993-Nov
Reference Count: N/A
Planned versus Unplanned Contrasts: Exactly Why Planned Contrasts Tend To Have More Power against Type II Error.
The literature is reviewed regarding the difference between planned contrasts, OVA and unplanned contrasts. The relationship between statistical power of a test method and Type I, Type II error rates is first explored to provide a framework for the discussion. The concepts and formulation of contrast, orthogonal and non-orthogonal contrasts are introduced. It is argued that planned contrasts are confused on thoughtful research questions of interest and reflect researchers' rational anticipation. An OVA test or unplanned contrasts, on the other hand, do not provide desired information in many situations. It is also explained that, to control for the possible inflated error rate for unplanned contrasts which usually test a large number of hypotheses, wanted or unwanted, some Bonferroni type of corrections are invoked. It is these corrections, usually built into statistical tables, that reduce the power of unplanned contrasts. This is demonstrated through a comparison of the critical values for planned contrasts and for some popular unplanned contrasts. (Contains 27 references.) (Author)
Publication Type: Reports - Evaluative; Speeches/Meeting Papers
Education Level: N/A
Authoring Institution: N/A
Identifiers: Planned Comparisons; Type II Errors; Unplanned Contrasts
Note: Paper presented at the Annual Meeting of the Mid-South Educational Research Association (22nd, New Orleans, LA, November 9-12, 1993).