I appears that you have the logic backwards. P-values less than alpha (0.05 in your example) mean that you can reject the null hypothesis that the true mean difference is 0.
Also, for the 2 sided test you either compare the 1-tail p-value to alpha/2 or double the 1-tail p-value (what R does if you specify 2 tails) and compare that to alpha. It looks like you are comparing 2*p-value to alpha/2 which is over correcting (doing a 4-tailed test, hmm, need to think of an example where that makes sense). -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Beat Meier > Sent: Monday, February 01, 2010 2:37 AM > To: r-help@r-project.org > Subject: [R] Question about interpretation of paired t-tests > > hi there, > > i have a few questions about the correct interpretation of a paired t- > test. > (i don't think that this matters, but I'm using R 2.10.1 on Windows). > > to my questions: > > i've been using a lot of time about this minor concerns now and I hope > you can help me... > > I use one- and two-sided t-tests. My questons are on one side about how > R uses the hypothesis' and the second one is about p-vaues. First the > one-sided: > > > t.test(var1,var2,paired=T,alternative="g") > > Paired t-test > > data: var1 and var2 > t = 4.3456, df = 50, p-value = 3.401e-05 > alternative hypothesis: true difference in means is greater than 0 > 95 percent confidence interval: > 0.00825842 Inf > sample estimates: > mean of the differences > 0.01344257 > > This tests (H0-Hypothesis), whether the difference in means is smaller > or equal to 0? Here, the p-value is smaller than 0.05, which should > mean > that at a level of 95% one can't reject the H0-hypothesis? there is no > strong evidence for rejecting the hypothesis, that the difference in > means is smaller or equal to 0? > (Question is, if I write "alternative="g"", if this defines my > alternative hypothesis (difference in means > 0) which I couldn't > reject > if the p-value was >0.05?) > > The two-sided: > > t.test(var1,var2,paired=T,alternative="t") > > Paired t-test > > data: var1 and var2 > t = 4.3456, df = 50, p-value = 6.801e-05 > alternative hypothesis: true difference in means is not equal to 0 > 95 percent confidence interval: > 0.007229405 0.019655737 > sample estimates: > mean of the differences > 0.01344257 > > This tests, whether the difference in means is 0. p-value is smaller > than 0.025 (alpha/2) at a level of 95%, so the hypothesis that the > difference in means is 0 can't be rejected? > > > Thanks a lot, > B. Meier > > ______________________________________________ > 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. ______________________________________________ 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.
