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Hi, I am trying to estimate the sample size needed for the comparison of two groups on a certain measurement, given some previous data at hand. I find that the data collected does not follow a normal distribution, so I would like to use a non-parametric option for sample size calculation. I found the pwr package but I don't think it has this option and on the internet found that

...as experimentally established only if a properly designed experiment rarely fails to give this level of significance.? [Fisher RA (1926), ?The Arrangement of Field Experiments,? Journal of the Ministry of Agriculture of Great Britain, 33, 503-513.] See the selection of Fisher quotes at http://www.jerrydallal.com/lhsp/p05.htm . In contexts where a p <= 0.05 becomes more likely under the NULL (not the case if the experiment might just as well have been a random number generator), small P-values shift the weight of evidence. An alternative that is apriori highly unlikely takes a lot of shifting. Jo...