On 16-Jun-10 22:30:39, Worik R wrote: > I have two pairs of related vectors > x1,y1 > and > x2,y2 > > I wish to do a test for differences in means of x1 and y1, > ditto x2 and y2. > > I am getting odd results. I am not sure I am using 'pt' properly... > I have not included the raw vectors as they are long. > I am interested if I am using R properly... > >> c(length(x1), length(y1), length(x2), length(y2)) > [1] 3436 1619 2677 2378 > > First where the T-stat and the DF do not give the same result as > 't.test' when passed into 'pt' > >> t.1 <- t.test(x1, y1) >> 2 * pt(t.1$statistic, t.1$parameter) > t > 1.353946 >> t.1$p.value > [1] 0.646054 > > I would have thought these would have been the same. Like below.... > >> t.2 <- t.test(x2, y2) >> 2 * pt(t.2$statistic, t.2$parameter) > t > 0.8679732 >> t.2$p.value > [1] 0.8679732 > > This is what I expect. > > clearly I misunderstand some thing. What is it? > > cheers > Worik
The P-value is the tail-area (or the sum of the two tail-areas for a two-sided test). The value of pt() is the total probability to the left of the upper tail. Taking your results above: [1]: t.1 <- t.test(x1, y1) 2 * pt(t.1$statistic, t.1$parameter) # t # 1.353946 t.1$p.value # [1] 0.646054 The "t.1$p.value" result will (by default) be the two-tailed test, so one tail will have probability equal to half the P-value, while the value of pt() will be Prob(T <= t1$statistic). Hence the former will be 2*(1 - the latter) **provided the t-statistic is positive** -- otherwise, if the t-statistic is negative, the former is twice the latter. . Check: 2*(1 - 1.353946/2) # [1] 0.646054 2*(1 - 0.646054/2) # [1] 1.353946 So this indicates that the t-value (which you did not quote) was positive. [2]: t.2 <- t.test(x2, y2) 2 * pt(t.2$statistic, t.2$parameter) # t # 0.8679732 t.2$p.value # [1] 0.8679732 2*(1 - 0.8679732/2) # [1] 1.132027 (so no agreement), but: 2*(0.8679732/2) # 0.8679732 so here the t-value was negative. And that is the difference between thw two cases. Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861 Date: 17-Jun-10 Time: 13:43:37 ------------------------------ XFMail ------------------------------ ______________________________________________ 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.
