posted and e-mailed. On Mon, 06 Jan 2003 22:29:56 GMT, "Arthur J. Kendall" <[EMAIL PROTECTED]> wrote:
[ ... ] > > It looks as if the TI is using about 8.1 df. (or the harmonic mean of > [10,7] ) which gives fractional df. > I think you nailed that one. > > Since one SD is twice the other, the Levene F for non homogeneous > variances is about 4. So the "separate variance" interval estimate > would be more appropriate and would result in small differences. > - of course, you are not supposed to make the choice of test on this ad-hoc basis. To be more explicit: according to a couple of articles that I read (and I agreed on this point), it is a BAD practice to 'condition' your choice. You are not supposed to let the test of variances determine which t-test to believe. For equal Ns, the difference in tests is *very* slight. For unequal Ns, both the versions of t-tests mis-behave rather badly in their one-tail rejection rates. The separate-variance tests (there are a couple of versions) are minimally more robust than the Students t, but not enough (in my opinion) to justify favoring them. - Insist on both tests showing the same, or consider transformations. Your "best test" will come if you find the transformation that is both logical and normalizing. A rank order transformation (non parametric test) is usually available and convenient, and almost as good. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
