Dang, Jeff wrote: > Edstat, > > I have personally found that a lot of health researchers like to aggregate > normally distributed, continuous outcomes into dichotmous outcomes. In some > cases, this is done because the researcher is more familiar with dicohotmous > outcomes (disease/no disease) and seeks to interpret their results in terms > of odds ratios within a logistic regression. > > In some cases, this can be problematic because you lose information. For > instance those near the cut-off point are forced into one group or another. > Thus, you exaggerate the differences for some individuals. > > If anybody on the listserv can refer me to articles related to this problem > I would be most appreciative. Thank you. > > Jeff Dang
The following is from a thread that appeared on the same topic last year. I don't know if the links given are still active. Cheers, Bruce On 2/4/03 2:25 PM, "Wuensch, Karl L" <[ mailto:[EMAIL PROTECTED] [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 > <[ 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. > ---- Gary McClelland replied ------ 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 -- Bruce Weaver [EMAIL PROTECTED] www.angelfire.com/wv/bwhomedir/ . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
