On Thu, 23 Nov 2006, ONKELINX, Thierry wrote: > There is no such thing as an unpaired t-test. A t-test can be a paired, > one sample or two sample t-test. Since you want to compare the sample > against a given mean, you need a one sample t-test. You tried to do a > two sample test. That didn't work because you need at least two > observations in each group. > > x <- c(23,25,29,27,30,30) > t.test(x[-1], mu = x[1]) > > One Sample t-test > > data: x[-1] > t = 5.3634, df = 4, p-value = 0.005833 > alternative hypothesis: true mean is not equal to 23 > 95 percent confidence interval: > 25.50814 30.89186 > sample estimates: > mean of x > 28.2
That is not what Robin is asking for. He has an unpaired two-sample t-test in the terminology I understand, but is also assuming var.equal=TRUE, which is not the default in R and so needs to specified. It should be possible to do x <- c(23,25,29,27,30,30) t.test(x=x[1], y=x[-1], var.equal=TRUE) since you can make use of the pooled variance from the two groups. In fact, R does not currently allow that case. > -----Oorspronkelijk bericht----- > Van: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Namens Robin Hankin > Verzonden: donderdag 23 november 2006 14:12 > Aan: [EMAIL PROTECTED] > Onderwerp: [R] t.test() > > Hi > > I have a vector x of length n. I am interested in x[1] > being different from the other observations (ie x[-1]). > > My null hypothesis is that x[1] > is drawn from a Gaussian distribution of the same > mean as observations x[-1], which are assumed > to be iid Gaussian. The (unknown) variance > of x[1] is assumed to be the same as the > variance of x[-1]. > > > This should be an unpaired t-test. > > But > > > > x <- c(23,25,29,27,30,30) > > t.test(x=x[1] , y=x[-1]) > Error in t.test.default(x = x[1], y = x[-1]) : > not enough 'x' observations > > > > > > What arguments do I need to send to t.test() to test my null? > > > > > > > > -- > Robin Hankin > Uncertainty Analyst > National Oceanography Centre, Southampton > European Way, Southampton SO14 3ZH, UK > tel 023-8059-7743 > > ______________________________________________ > R-help@stat.math.ethz.ch 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@stat.math.ethz.ch 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. > -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ R-help@stat.math.ethz.ch 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.