Dear John,
Yes, that's great - thanks!
Andy
John Fox wrote:
Dear Andy,
Yes, the tetrachoric correlation is a special case of the polychoric
correlation when both factors are dichotomous.
The 95-percent confidence interval that you suggest might be adequate if the
sample size is sufficiently large and the correlation isn't too close to 0
or 1, but it is probably not in general terribly trustworthy.
I hope this helps,
John
--------------------------------
Original message:
Andy Fugard a.fugard at ed.ac.uk
Mon Sep 1 19:25:54 CEST 2008
Hi there,
Am I correct to believe that tetrachoric correlation is a special case
of polychoric correlation when there are only two levels to the ordered
factor? Thus it should be okay to use hetcor from the polycor package
to build a matrix of correlations for binary variables?
If this is true, how can one estimate 95% confidence intervals for the
correlations? My guess would be
mat = hetcor(dataframe)
mat$correlation - (1.96 * mat$std.errors)
mat$correlation + (1.96 * mat$std.errors)
Thanks,
Andy
--
Andy Fugard, Postgraduate Research Student
Psychology (Room S6), The University of Edinburgh,
7 George Square, Edinburgh EH8 9JZ, UK
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