On 10 Dec 2003 07:30:40 -0800, [EMAIL PROTECTED] (Donald Burrill) wrote: > So far all responses have appeared to assume that the data represent > left and right ovaries on the SAME women. But this cannot be the case, > because the total number of left ovaries in the data is 132, and of > right ovaries 103. So: before we can offer useful advice, we need to > know whether the pathology being documented occurs, when it occurs, only > in one of a woman's ovaries (for these data, anyway), or whether some of > the 235 instances of pathology represent both ovaries for some of the > women. If there are any of these last, those are the only cases for > which one has paired data. [ snip, rest]
It took me a few minutes to figure out the complications of these data, when there is or is-not much pairing. I was ignoring it, yes, because I was assuming (as DFB says) that there was a lot of pairing, and a lot of cases with zero pathology. Those things don't have to be true. If the L and R instances are disjunct, then the paired t-test would take place with a negative correlation, and the absolute correlation would be *larger* for a smaller total N. I am not sure how much that affects the resulting t-test, carried out as a paired t. (Is this only a minor, theoretical concern? How weird do the extremes have to be? - I don't have the time or interest to check this right now.) However, the Fisher's test, using binomial proportions, still works. You note which is *worse*, L or R, and you can count that, whether or not bad ovaries are on the same woman. You throw away more information about severity, if there is not a lot of actual disease-pairing, and a high correlation, L with R, but the test is still good. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html "Taxes are the price we pay for civilization." . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
