[visionlist] analyzing medians
andrew.leber at unh.edu
Wed Aug 4 17:48:24 GMT 2010
Hi Todd -- Taking the means of medians seems "perfectly okay."
Consider that you're already using means of medians when reporting
group-level data. The reason you and your collaborator saw a subtle
difference is probably because you don't have an equal number of
trials per condition in the levels of variable D, thus over-
representing some levels of D while under-representing others when
computing a single median (vs. taking the mean of medians).
Andrew B. Leber, Ph.D.
Assistant Professor of Psychology
University of New Hampshire
On Aug 4, 2010, at 12:20 PM, Todd S. Horowitz wrote:
> I have a puzzle about analyzing RT data. I prefer to use medians
> rather than means, because I am suspicious of all of the various
> data trimming procedures. However, medians seem to be creating some
> problems when I run ANOVAs on the data.
> I'm working with some data. Let's say there are 4 factors, A, B, C,
> and D. However, the critical analyses collapse over the levels of
> factor D. My collaborator sent me an analysis where she took the
> ABCD medians, then collapsed by taking the means of those medians
> across factor D, then running the ANOVA. I decided that was
> incorrect, and directly computed the ABC medians from the raw data,
> then ran an ANOVA. The results were subtly different, pushing the 3-
> way interaction across the p = .05 line. However, it then ocurred to
> me that the ANOVA does just the same thing as what my collaborator
> did: the A main effect takes the means of the medians. If I were to
> directly compute the A medians from the raw data, and run a one-way
> ANOVA, I would probably get subtly different results from the ANOVA
> on the ABC medians.
> So, what's the correct approach to this analysis? Do I give up and
> work with means? Always recompute the medians from the raw data for
> each effect separately? Or is it perfectly OK to just take the mean
> of medians?
> Todd S. Horowitz, PhD
> Assistant Professor of Ophthalmology
> Harvard Medical School
> Associate Director
> Visual Attention Lab
> Brigham & Women's Hospital
> 64 Sidney Street, Suite 170
> Cambridge, MA 02139
> phone: (617) 768-8813
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