ERIC Number: ED426092
Record Type: RIE
Publication Date: 1999-Jan
Reference Count: N/A
Planned Contrasts: An Overview of Comparison Methods.
Contrasts or comparisons can be used to investigate specific differences between means. Contrasts, as explained by B. Thompson (1985, 1994) are coding vectors that mathematically express hypotheses. The most basic categories of contrasts are planned and unplanned. The purpose of this paper is to explain the relative advantages of using planned contrasts rather than unplanned contrasts and to illustrate several different planned contrasts that are available. Planned contrasts are designed to test predetermined specific hypotheses and have more statistical power against Type II errors than unplanned comparisons. The most common distinction among planned contrasts is whether they are orthogonal (uncorrelated) or nonorthogonal. A hypothetical data set illustrates the difference between the two. Descriptions are also given of more specific coding methods for contrasts. These methods include trend versus nontrend analyses, dummy coding, and effect coding. The use of planned contrasts is often a good alternative to traditional analysis of variance. (Contains 4 tables and 18 references.) (SLD)
Publication Type: Reports - Descriptive; Speeches/Meeting Papers
Education Level: N/A
Authoring Institution: N/A
Identifiers: Orthogonal Comparison; Planned Comparisons; Type II Errors
Note: Paper presented at the Annual Meeting of the Southwest Educational Research Association (San Antonio, TX, January 21-23, 1999).