On 2/4/03 2:25 PM, in article 3B4D60A9CF29C349B185DC404F988D1002290F13@MAIL2A, "Wuensch, Karl L" <[EMAIL PROTECTED]> wrote:
> Thanks to all who have participated in the discussion on > dichotomization. I have logged selected parts of the discussion into a > document for both my students and my colleagues to read. It is available at > http://core.ecu.edu/psyc/wuenschk/StatHelp/Dichot-Not.doc > <http://core.ecu.edu/psyc/wuenschk/StatHelp/Dichot-Not.doc> . If any of > you would rather not have your comments included in that document, please > let me know. > I've put together a Java applet that illustrates the issues. http://psych.colorado.edu/~mcclella/MedianSplit/ There is a slider that allows you to move the values of X smoothly from the continuous analysis to the dichotomized analysis. Doing so you can see that as the variance of X is thereby reduced, the correlation is reduced, but the estimate of the slope remains unbiased. The effect of dichotomization is simply to reduce statistical power. If X is normally distributed, the effect of dichotomization is equivalent to discarding a random 50% (approximately) of one's observations. I can't imagine why anyone would want to do that. Note: to use the above applet, PC users will need to have downloaded Sun's Java plug-in to replace the crippled Java that Microsoft distributes. Mac users will need to be using OS X. gary . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
