The ANOVA assumes equal variances in the groups. Suppose groups 5 and 6 had much lower variances than groups 1 to 4, and group 6 had a different mean from the other 5 (which were about equal)?
Given how small the groups apperat to be, this could happen. On Tue, 21 Oct 2003, Bill Shipley wrote: > Hello. I have come across a curious result that I cannot explain. > Hopefully, someone can explain this. I am doing a 1-way ANOVA with 6 > groups (example: summary(aov(y~A)) with A having 6 levels). I get an F > of 0.899 with 5 and 15 df (p=0.51). I then do the same analysis but > using data only corresponding to groups 5 and 6. This is, of course, > equivalent to a t-test. I now get an F of 142.3 with 1 and 3 degrees of > freedom and a null probability of 0.001. I know that multiple > comparisons changes the model-wise error rate, but even if I did all 15 > comparisons of the 6 groups, the Bonferroni correction to a 5% alpha is > 0.003, yet the Bonferroni correction gives conservative rejection > levels. > > How can such a result occur? Any clues would be helpful. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help