Thanks Jim and Ben for your replies, Reading further about data normalization found shapiro.test. I understand that if the p-value is smaller than 0.05 then the data isn't normal, I just don't understand what the "W" means.
Hi Felipe, Here's one way: library(nortest) if(sf.test(fishlength)$p.value>0.05) t.test(fishlength) else wilcox.test(fishlength) Jim > Felipe Carrillo <mazatlanmexico <at> yahoo.com> > writes: > > > > > Hello all: > > Is there a way to detect in R if a dataset is normally > distributed or skewed > without graphically seeing it? > > The reason I want to be able to do this is because I > have developed and > application with Visual Basic where > > Word,Access and Excel "talk" to each other > and I want to integrate R to this > application to estimate > > confidence intervals on fish sizes (mm). I basically > want to automate the > process from Excel by detecting > > if my data has a normal distribution then use t.test, > but if my data is skewed > then use wilcox.test. > > Something like the pseudo code below: > > > > fishlength <- > c(35,32,37,39,42,45,37,36,35,34,40,42,41,50) > > if fishlength= "normally distributed" > then > > t.test(fishlength) > > else > > wilcox.text(fishlength) > > > > I hope this isn't very confussing > > > > Felipe D. Carrillo > > Supervisory Fishery Biologist > > Department of the Interior > > US Fish & Wildlife Service > > California, USA > > > There's a whole package (nortest) devoted to tests of > normality, > BUT: I would suggest that your procedure is not a good > idea. > It's often hard to detect non-normality, and "fail > to reject" > shouldn't mean "accept". If you're > concerned about non-normality, > you should probably just use the Wilcoxon test all the time > (it has about 95% of the power of the t-test if the data > are > normal: http://en.wikipedia.org/wiki/Mann-Whitney_U ), or > use robust statistics (e.g. rlm in the MASS package). > > Ben Bolker ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
