At 10:50 AM 3/27/02 -0500, John W. Kulig wrote:
>JP wrote: > > > I have been unable to find an adequate explanation of exactly what is the > > difference between an interclass and in intraclass correlation, and the > > circumstances in which you would choose either. > > Ian Kestin > >This is in answer to the second half of your question: Use the interclass >when you are correlating two different things, such as height and weight. You >can go through the entire data set of paired scores and it is obvious which >number in each pair is a height (call it A) and which is a weight (B). >Intraclass correlations are used when there you cannot do this. They are used >(as one example) to get correlations for twins. If you had a set of IQ scores >from twins, each pair of IQ scores is from a twin pair - but there is no >basis for assigning one A and the other B. Each member of a twin duo could as >easily be thrown into column A as column B. to expand on this a bit without going into stat funk detail, think about some strategies one might look at when you want to examine the "relationship" between two columns of scores ... when we have (say) twin data to make it very simple, say we have 4 twin pairs on IQs ... IQ A IQ B 103 109 88 84 128 119 97 92 now, we could do a regular PPM r on the data we have ... r=.928 but, since there is no necessary logical reason why the twins are ordered across the lines as they are ... the set of data could have been (and made just as much sense) 103 109 84 88 128 119 92 97 in this case, the r = .964 well, which is right? what if we rearranged the data in all possible IQA and IQB configurations ... did all the rs ... and took an average? we might say that this is the "typical" r you might get when correlating the two columns of values ... no matter which of the pair comes first or second another conceptual way would be the following the notion above is that the pair of values on a line ... that is, the twin PAIR ... should not be as different as, differences we might see DOWN ONE COLUMN OR THE OTHER COLUMN IF there is something going on in terms of genetics. that is ... the between column variance (by rows) should be rather small ... compared to the WITHin column variance in the extreme case, the two values in each of the columns might be the same for both columns like: 103 103 84 84 128 128 97 97 here we see no BETWEEN column variability ... across the rows ... but, clearly, there is within column variability now, this would not be usual of course but, does set the upper limit thus, some comparison of between column variation (across the rows) versus within column variation ... can help to examine the issue of relationship between the columns of data typically, scenarios like this might find the paired columns being for twins ... versus the paired columns being for non twin siblings ... and, in the analysis above, if the comparison of between column (across the rows) variances versus within column variances is different in the two cases ... then, we say that the role of genetics (perhaps) is the reason why the relationship between the sets of data are different >-- >------------------------------------------------------------------ >John W. Kulig [EMAIL PROTECTED] >Department of Psychology http://oz.plymouth.edu/~kulig >Plymouth State College tel: (603) 535-2468 >Plymouth NH USA 03264 fax: (603) 535-2412 >------------------------------------------------------------------ >"Kane to kal� ke r�chto sto yal�." >(Do a good deed and cast it to the sea) > Ancient Greek saying >------------------------------------------------------------------ > > >. >. >================================================================= >Instructions for joining and leaving this list, remarks about the >problem of INAPPROPRIATE MESSAGES, and archives are available at: >. http://jse.stat.ncsu.edu/ . >================================================================= Dennis Roberts, 208 Cedar Bldg., University Park PA 16802 <Emailto: [EMAIL PROTECTED]> WWW: http://roberts.ed.psu.edu/users/droberts/drober~1.htm AC 8148632401 . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
