Hello Thanks all for your input!
On Fri, Oct 1, 2010 at 3:55 AM, RICHARD M. HEIBERGER <[email protected]> wrote: > The t-value in the cor.test is already scaled for 30 df by the time it > is printed. Giving an n-value scales it again, by dividing the > standard deviation by sqrt(n). Since you are already in the standard > t-scaling, n=1 is the correct value for your example. > This is the graph you are looking for: > > normal.and.t.dist(obs.mean=5.5651, deg.freedom=30, alpha.right=.025, > Use.alpha.left=TRUE, Use.obs.mean=TRUE, xmin=-6) > This is indeed the graph that I was looking for. I was assuming that, since I knew both df and t, I should specify both, but I was obviously wrong. Also, I prefer this graph to the below because it's cleaner and should raise less questions. Thanks a lot for the helpful functions. Best regards Liviu > The above graph does not specifiy n, and therefore shows > "Standard t Density", and only the t axis. The green 5.565 is the observed > t-value, and the -5.565 is the boundary of the p-value region on the other > side. > We need both sides because the cor.test said the alternative hypothesis is > two-sided. > > Here is almost the same graph, this time with n=1. Now it shows "t density" > with > standard error of x.bar and the x.bar axis as well as the t axis. > normal.and.t.dist(obs.mean=5.5651, deg.freedom=30, alpha.right=.025, > Use.alpha.left=TRUE, Use.obs.mean=TRUE, xmin=-6, n=1) > In this example with n=1, the x.bar scale and the t scale are identical. > The blue +/- 2.042 are the boundaries of the rejection region in x.bar > units. > On your original graph, simplified here > > normal.and.t.dist(obs.mean=5.5651, deg.freedom=30, alpha.right=0.025, > Use.alpha.left=TRUE, Use.obs.mean=TRUE, n=32) > > the blue values are in the x.bar scale (as indicated on the graph), > asssuming that the observed mean x.bar=5.5651 comes from a > distribution of x.bar with standard deviation 1/sqrt(32). > Scale that to the t-scale with 5.5651/(1/sqrt(32)) = 31.48096 and that > is the number you were querying. > > Expand the graph to fill the screen and you will see that the right boundary > of the rejection region is suppressed by R to avoid overprinting. > If you extend the graph to the left, with xmin=-6, > > normal.and.t.dist(obs.mean=5.5651, deg.freedom=30, alpha.right=0.025, > Use.alpha.left=TRUE, Use.obs.mean=TRUE, n=32, xmin=-6) > > then you will see that the left boundary of the p-value region -31.481 > is also displayed. > Rich > -- Do you know how to read? http://www.alienetworks.com/srtest.cfm http://goodies.xfce.org/projects/applications/xfce4-dict#speed-reader Do you know how to write? http://garbl.home.comcast.net/~garbl/stylemanual/e.htm#e-mail _______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-sig-teaching
