One can show algebraically that when the sample sizes are equal, the pooled variance t test and separate variance t test are equivalent. So that if you agree with the work by Donald Zimmerman below and Karl's thinking, there is no reason to ever use the pooled variance t test.
Howard Kaplon "Karl L. Wuensch" wrote: > Monte Carlo work by Donald W. Zimmerman (Some properties of > preliminary tests of equality of variances in the two-sample location > problem, Journal of General Psychology, 1996, 123, 217-231) has > indicated that two stage testing (comparing the variances to determine > whether to conduct a pooled test or a separate variances test) is not > a good procedure, especially when the sample sizes differ greatly (3 > or 4 times as many subjects in one group than in the other, in which > case the pooled test performs poorly even when the ratio of variances > is as small as 1.5).Zimmerman's advice is that the separate variances > t should be applied unconditionally whenever sample sizes are > unequal.Given the results of his Monte Carlo study, I think this is > good advice, and I advise my students to adopt the practice of using > the separate variances test whenever they have unequal sample sizes.I > still believe that the pooled test may be appropriate (and more > powerful) when the sample sizes are nearly equal and the variances not > greatly heterogeneous, but carefully defining "nearly equal sample > sizes" and "not greatly heterogeneous variances" is not something I > care to tackle. > > ------------------------------------------------------------- > Karl L. Wuensch, Department of Psychology, > East Carolina University, Greenville NC 27858-4353 > Voice: 252-328-4102 Fax: 252-328-6283 > [EMAIL PROTECTED] > http://core.ecu.edu/psyc/wuenschk/klw.htm > > > -- ------------------------------------------------------- | Howard S. Kaplon | Mathematics Department | | Phone: 410-704-3087 | Towson University | | FAX: 410-704-4149 | 8000 York Road | | | Towson, Maryland 21252 | | | United States of America | | | | E-mail: [EMAIL PROTECTED] | ------------------------------------------------------- . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
