[R] Polychoric and tetrachoric correlation

[Andy Fugard]

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
> 


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Andy Fugard, Postgraduate Research Student
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