It is NOT true that with equal sample sizes the separate variances and the pooled variances t tests are equivalent (except in one case, where the sample variances are identical). The computed values of t will be identical with equal sample sizes, but the degrees of freedom (and thus the value of p and the width of the confidence interval) will not. With equal sample sizes one still must make an assumption of homogeneity of variance to use the pooled degrees of freedom. Zimmerman did not seem to think it unreasonable to consider the observed variances when deciding between pooled and separate degrees of freedom when the sample sizes are equal.
Karl W. ----- Original Message ----- From: "Howard Kaplon" <[EMAIL PROTECTED]> To: "Karl L. Wuensch" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]> Sent: Sunday, January 12, 2003 9:03 PM Subject: Re: When to use a separate variances standard error 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 . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
