>Greetings, > I'm trying to estimate the degree of agreement between mothers and >fathers in 172 families which I would like to use in further analyses. >Both parents responded to 14 items using a 4-point Likert response >scale. I'm using an intraclass correlation (ICC) (as opposed to a >Pearson correlation) to measure the agreement across the 14 items for >each set of parents. I've downloaded the SAS macro developed by Robert >Hamer that implements the 6 types of ICCs identified by Shrout and >Fleiss in their Psychological Bulletin article. The SAS macro uses Proc >Glm to derive the variance components used to compute the particular >ICCs. > >About 15 of the 172 ICCs are negative which I would like to understand >better since it doesn't seem possible to have a negative variance. In >fact, one of the negative ICCs is a value over 1 (-1.42). >Unfortunately, I can't find my copy of the Shrout and Fleiss article at >the moment to see if they discuss this possibly. I would appreciate any >ideas on how to deal with the negative ICCs or references to examine. >TIA, >Tom Bohman >bohman@pclink.com or tmb@wilder.org > > I don't know about the macro, but I would guess the variance components are estimated by the ANOVA method/methods of momets. It is quite possible to get negative estimates by these methods. This is indication that the variance is, in fact zero or very small, and/or that the sample size is small, so you get a large standard error of your estimate.

Hans-Peter _______________________________________________________________________ Hans-Peter Piepho Institut f. Nutzpflanzenkunde WWW: http://www.wiz.uni-kassel.de/fts/ Universitaet Kassel Mail: piepho@wiz.uni-kassel.de Steinstrasse 19 Fax: +49 5542 98 1230 37213 Witzenhausen, Germany Phone: +49 5542 98 1248