On Fri, 25 Jul 2003 18:03:52 +0200, Peter Frank <[EMAIL PROTECTED]> wrote:
> Rich Ulrich <[EMAIL PROTECTED]> wrote: > > >On Wed, 23 Jul 2003 17:50:24 +0200, Peter Frank > ><[EMAIL PROTECTED]> wrote: > > > >> Which other study designs yield paired data other than before/after > >> experiments? > > > >Left-right. > OK. I can pair data from an individual's right hand with data from his > left hand. > > >Male-female. > How can I specifically pair data from males and with data from > females? >From each litter of puppies, take one (at random) of each. The fact of litter-membership defines the 'correlation' > >Top-bottom. > >Hot-cold. > If these are factor to be examined but not the subjects, I understand > it, otherwise I don't (see above). > Is there something that defines a 'correlation' between the members? - if so, then you might treat them as paired for an analysis. This is a somewhat subtle point. Correlation exists with-respect-to something outside. One hundred trials with my right hand might be considered uncorrelated -- if there was an experiment designed within those 100 trials. However, they are self-similar compared to trials with my left hand. And both my hands are self-similar (for me) compared to trials done with by someone else. The classic studies of 'case-control' often started with a limited number of 'cases' and matched each Case with the candidate among 'controls' who is the same (or closest) with respect (say) to age and gender. These case-control studies are less popular today because (a) bad historical matches yielded some bad analyses; and (b) the statistical control by covariates tends to be more powerful; and (c) statistical control by covariates is more meaningful, since you can have tests on those covariates. 'Case-control' is not a bad idea for collecting data, at times when you don't want the samples to have disproportions of various characteristics -- I am thinking especially of cases where it is cheap and easy to screen for potential 'controls' but expensive to actually carry out the exams to put them in the sample. Otherwise, you should add all your recruits to the Control, to boost its N and representativeness. But the statistical analyses should be done as Case-control only when the pairs are *highly* similar. Most often that I think of, those would be pairs of family members. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html "Taxes are the price we pay for civilization." Justice Holmes. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
