On Aug 15, 2010, at 9:05 AM, R Help wrote: > Hello List, > > I'm trying to do a paired t-test, and I'm wondering if it's consistent > with equations. I have a dataset that has a response and two > treatments (here's an example): > > ID trt order resp > 17 1 0 1 0.0037513592 > 18 2 0 1 0.0118723051 > 19 4 0 1 0.0002610251 > 20 5 0 1 -0.0077951450 > 21 6 0 1 0.0022339952 > 22 7 0 2 0.0235195453 > > The subjects were randomized and assigned to receive either the > treatment or the placebo first, then the other. I know I'll > eventually have to move on to a GLM or something that incorporates the > order, but for now I wanted to start with a simple t.test. My problem > is that, if I get the responses into two vectors x and y (sorted by > ID) and do a t.test, and then compare that to a formula t.test, they > aren't the same. > >> t.test(x,y,paired=TRUE) > > Paired t-test > > data: x and y > t = -0.3492, df = 15, p-value = 0.7318 > alternative hypothesis: true difference in means is not equal to 0 > 95 percent confidence interval: > -0.010446921 0.007505966 > sample estimates: > mean of the differences > -0.001470477 > >> t.test(resp~trt,data=dat1[[3]],paired=TRUE) > > Paired t-test > > data: resp by trt > t = -0.3182, df = 15, p-value = 0.7547 > alternative hypothesis: true difference in means is not equal to 0 > 95 percent confidence interval: > -0.007096678 0.005253173 > sample estimates: > mean of the differences > -0.0009217521 > > What I'm assuming is that the equation isn't retaining the inherent > order of the dataset, so the pairing isn't matching up (even though > the dataset is ordered by ID). Is there a way to make the t.test > retain the correct ordering? > > Thanks, > Sam
See this thread from just 2 days ago: https://stat.ethz.ch/pipermail/r-help/2010-August/249068.html perhaps focusing on Thomas' reply, which is the next post in the thread. Bottom line, don't use the formula method for a paired t test. HTH, Marc Schwartz ______________________________________________ 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.
