Except that t-test does not assume that *observations* are normally distributed, nor that variances are equal.
Avoid non-parametric tests: they assume too much of data properties. For var.equal assumption in t.test, see ?t.test. Cheers, Jari Oksanen ________________________________________ From: r-sig-ecology-boun...@r-project.org [r-sig-ecology-boun...@r-project.org] on behalf of Richard Boyce [boy...@nku.edu] Sent: 24 March 2014 13:23 To: r-sig-ecology@r-project.org Subject: Re: [R-sig-eco] report out by t.test Mike, There is no way that your data meet the assumptions of a t-test (normal distributions, equal variance). A nonparametric Mann-Whitney (aka Wilcoxon) test is much better suited to your data. Here's what I got when I ran it: Q<-c(13,0,10,2,0,0,1,0,0,1,5) WD<-c(0,0,1,0,0,0,0,0,0,0,1) wilcox.test(Q,WD) Wilcoxon rank sum test with continuity correction data: Q and WD W = 86.5, p-value = 0.05119 alternative hypothesis: true location shift is not equal to 0 Warning message: In wilcox.test.default(Q, WD) : cannot compute exact p-value with ties This has a p-value quite close to 0.05, giving some evidence that there's a difference between your groups. Note that this you have different null and alternative hypothesis: groups are the same vs. groups are different. Rick Boyce On Mar 24, 2014, at 7:00 AM, r-sig-ecology-requ...@r-project.org<mailto:r-sig-ecology-requ...@r-project.org> wrote: Message: 1 Date: Sun, 23 Mar 2014 14:21:41 -0700 From: Michael Marsh <sw...@blarg.net<mailto:sw...@blarg.net>> To: r-sig-ecology@r-project.org<mailto:r-sig-ecology@r-project.org> Subject: [R-sig-eco] report out by t.test Message-ID: <532f5065.7030...@blarg.net<mailto:532f5065.7030...@blarg.net>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed I test differences between frequency of hits of exotic annual forbs in plots on two sites, Q and WD. Q<-c(13,0,10,2,0,0,1,0,0,1,5) WD<-c(0,0,1,0,0,0,0,0,0,0,1) t.test(Q,WD) Welch Two Sample t-test data: Q and WD t = 1.9807, df = 10.158, p-value = 0.07533 alternative hypothesis: true difference in means is not equal to 0 95 percent confidence interval: -0.3342006 5.7887460 sample estimates: mean of x mean of y 2.9090909 0.1818182 The p-value is greater than 0.05, thus does not reach the 95% confidence level, yet the difference in means is reported as not equal to 0. Am I encountering a one-sided versus two sided comparison that I don't understand, or is ther another explanation? Mike Marsh ================================ Richard L. Boyce, Ph.D. Director, Environmental Science Program Professor Department of Biological Sciences, SC 150 Northern Kentucky University Nunn Drive Highland Heights, KY 41099 USA 859-572-1407 (tel.) 859-572-5639 (fax) boy...@nku.edu<mailto:boy...@nku.edu> http://www.nku.edu/~boycer/ ================================= "One of the advantages of being disorderly is that one is constantly making exciting discoveries." - A.A. Milne [[alternative HTML version deleted]] _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology