[R] Power analysis for MANOVA?

Rick Bilonick rab at nauticom.net
Mon Feb 2 03:05:51 CET 2009

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

Rick B.

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