Bruce Weaver <[EMAIL PROTECTED]> wrote:
: Paul's post reminded me of something I read in Keppel's Design and : Analysis. Here's an excerpt from my notes on ANCOVA: : the analysis of covariance is more precise with correlations greater than : .6. Since we rarely obtain correlations of this latter magnitude in the : behavioral sciences, we will not find a unique advantage in the analysis : of covariance in most research applications. I've NEVER seen a pre-post correlation less than .4 : Keppel (1982, p. 513) also prefers the Treatments X Blocks design : to ANCOVA on the grounds that the underlying assumptions are less : stringent: He's wrong in the random assignment case; the assumptions are essentially the same. The ANCOVA estimates are unbiased without any assumptions and without assumption test exactly the same hypothesis as the simple t test or the test of change scores (or treatments x blocks) test ================================================================= Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =================================================================