Power analysis for MANOVA?
On Wed, 2009-01-28 at 21:21 +0100, Stephan Kolassa wrote:
Hi Adam, first: I really don't know much about MANOVA, so I sadly can't help you without learning about it an Pillai's V... which I would be glad to do, but I really don't have the time right now. Sorry! Second: you seem to be doing a kind of "post-hoc power analysis", "my result isn't significant, perhaps that's due to low power? Let's look at the power of my experiment!" My impression is that "post-hoc power analysis" and its interpretation is, shall we say, not entirely accepted within the statistical community, see: Hoenig, J. M., & Heisey, D. M. (2001, February). The abuse of power: The pervasive fallacy of power calculations for data analysis. The American Statistician, 55 (1), 1-6 And this: http://staff.pubhealth.ku.dk/~bxc/SDC-courses/power.pdf However, I am sure that lots of people can discuss this more competently than me... Best wishes Stephan
The point of the article was that doing a so-called "retrospective" power analysis leads to logical contradictions with respect to the confidence intervals and p-values from the analysis of the data. In other words, DON'T DO IT! All the information is contained in the confidence intervals which are based on the observed data - an after the fact "power analysis" cannot provide any insight - it's not data analysis. Rick B.