Karl L. Wuensch writes: > My colleague's query was, however, "if you are going to > dichotomize a continuous subject variable and compare the resulting > two groups on a second continuous variable, even though that is > not generally a good idea, is it more useful to use a median split > (upper half vs lower half) or to compare the tails (such as upper > third versus lower third)?" I suggested to my colleague that this > would depend, in part, on the form of the relationship between the > two continuous variables (not necessarily strictly linear), > and reminded him that throwing out the middle of the > distribution would reduce N and thus might reduce power too. My > colleague confessed that he was fishing for a citation to justify > having done something that I told him earlier was not a good thing to > do. ;-)
There's a trade-off here. By removing the middle third, you increase the separation of the two groups, which is good, while at the same time reducing the sample size, which is bad. Usually the trade-off is good. It's not too hard to show that the loss of information is related to the correlation between the original variable and a new variable which equals -1, 0, or +1 depending on which third of the data you are in. For most data sets, this is slightly better than the correlation between the original variable and a new variable which equals -1 for the first half and +1 for the second half. Tukey came up with a simple regression fit that involved removing the middle third of the data. So you have some precedent for this approach. I would not be as critical as some of the others on the list. Sometimes a categorical variable is easier to interpret. A lot of dietary research, for example, looks at the highest quintile of fat consumption and compares it to the lowest quintile. I can visualize those two groups pretty well. Furthermore, categorization mitigates some of the problems caused by measurement error. If I were doing it myself, I would almost never dichotomize. But I wouldn't be too upset if someone else did it, especially if the data set were already quite large. Steve Simon, [EMAIL PROTECTED], Standard Disclaimer. The STATS web page has moved to http://www.childrens-mercy.org/stats. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
