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From | Yashin <yashin5@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: Polychoric PCA error message |
Date | Wed, 30 Jan 2013 21:54:38 -0500 |
Dear Dr. Kolenikov, Many thanks for your prompt and helpful response. I am still digesting the details of your explanation below; what I understood more generally is that while the Savalei paper recommends substituting 0.5 for zero in contingency tables as an adjustment in this procedure, --polychoric-- does not include such an adjustment in its code. I hope I understood correctly. If so, it appears that I have at least a couple of options--to drop variables that have been leading to zeros in contingency table cells, or to use another procedure, such as ordinal PCA. Do you happen to know of other options I could consider? Also, I encountered a situation that was unexpected for me. I described in my previous email, and I copied it below--is this surprising to you as well? If not might you recommend a reference that might help me understand this better? > 2) When I run the polychoric with only the dichotomous variables, and > then with the same variables plus the additional 5 variables described > above (ordinal and continuous), I get different correlation > coefficients in the correlation matrix for the same variable pairs. > How could this be? Sometimes the values are similar and yet different, > and in other cases the values are quite different (some of the > correlations > 0.9 when binary, ordinal and continuous variables are > included in the matrix become zero when only binary variables are > included in the matrix). Thank you for your help, Yashin ________________________________