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Analysis of Variance (ANOVA) Models in Lower Extremity WoundsResearch Institute, St. Luke's Hospital, Bethlehem, PA, USA, ReedJ{at}slhn.org Consider a study in which 2 new treatments are being compared with a control group. One way to compare outcomes would simply be to compare the 2 treatments with the control and the 2 treatments against each using 3 Student t tests (t test). If we were to compare 4 treatment groups, then we would need to use 6 t tests. The difficulty with using multiple t tests is that as the number of groups increases, so will the likelihood of finding a difference between any pair of groups simply by change when no real difference exists by definition a Type I error. If we were to perform 3 separate t tests each at α = .05, the experimental error rate increases to .14. As the number of multiple t tests increases, the experiment-wise error rate increases rather rapidly. The solution to the experimental error rate problem is to use analysis of variance (ANOVA) methods. Three basic ANOVA designs are reviewed that give hypothetical examples drawn from the literature to illustrate single-factor ANOVA, repeated measures ANOVA, and randomized block ANOVA. `No frills' SPSS or SAS code for each of these designs and examples used are available from the author on request.
Key Words: analysis of variance • single-factor analysis of variance • repeated measures analysis of variance • randomized block analysis of variance
The International Journal of Lower Extremity Wounds, Vol. 2, No. 2,
87-95 (2003) |
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