HI, Jari The default in the Welch t test (an adaptive student's t test) doesn't assume equal variance; but student's t-test do assume normal distribution.
Cheers Li -----Original Message----- From: r-sig-ecology-boun...@r-project.org [mailto:r-sig-ecology-boun...@r-project.org] On Behalf Of Jari Oksanen Sent: Monday, 24 March 2014 10:43 PM To: Richard Boyce; r-sig-ecology@r-project.org Subject: Re: [R-sig-eco] report out by t.test 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 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- This email is intended for the addressee(s) named and may contain confidential and/or privileged information. If you are not the intended recipient, please notify the sender and then delete it immediately. Any views expressed in this email are those of the individual sender except where the sender expressly and with authority states them to be the views of the Office of Environment and Heritage, NSW Department of Premier and Cabinet. PLEASE CONSIDER THE ENVIRONMENT BEFORE PRINTING THIS EMAIL _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
