Many thanks go to Robert Newcombe, Roger Newson, [log in to unmask] and Neal Alexander who replied to my mail re. multiple comparisons.
If I may, I would like to ask a further question.
Suppose I am conducting a repeated measures analysis. Hypothetically I have one within subjects factor (with 5 levels) and one between subjects factor (with 3 levels) - say there are n different subjects in each level of the between subjects factor. When I run the analysis I get a significant interaction and so I decide to conduct 'simple main effects' tests. I aim to establish if there is a difference between the 3 levels of the between subjects factor at each level of the within subjects factor. I realise that there are many ways to do this, but I decide to do five 1-way ANOVAs...one for each level of the within subjects factor. As we get 5 p-values I can apply, for example, the traditional Bonferroni correction (i.e. comparing each p value to 0.05/5). Let's assume that the results of the 5 ANOVAs are significant at the table wide 0.05 level. Say if I want to subsequently look at Bonferroni post hoc tests for each of these 5 ANOVAs to establish, for each within subjects factor level, which pairs of the between subjects factor levels are different. As there are 3 between subjects factor levels there will be 3 pairs of levels which are compared (1 vs 3,1 vs 2,2 vs 3) for each within subjects factor level. SPSS has an option to do these post hoc tests but in one SPSS text I have read it implies that since we have used 0.05/5=0.01 when looking at the significance of the 5 ANOVAs, we should also choose a 'significance level' of 0.01 in the 'Post Hoc' dialogue box (Analyze>Compare Means>One Way ANOVA) thus, for the Bonferroni post hoc tests for each 1-way ANOVA, each comparison is tested at the alpha level for the ANOVA divided by the number of comparisons; for our example, 0.01/3....... I'd appreciate your views on this.
Many thanks again for your advice on his matter ,
Kind Regards,
Kim
Dr Kim Pearce PhD, CStat
Senior Statistician
Haematological Sciences
Institute of Cellular Medicine
William Leech Building
Medical School
Newcastle University
Framlington Place
Newcastle upon Tyne
NE2 4HH
Tel: (0044) (0)191 282 0451
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