item-scale correlation correcting for overlap

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Contributor lei
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Posts: 37

item-scale correlation correcting for overlap

Hi,

Does anyone know how to correct item-scale correlation for overlap? I'm using Spearman correlation at this point.

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Re: item-scale correlation correcting for overlap

Do you mean from covariance matrix to correlation matrix ?

Contributor lei
Contributor
Posts: 37

Re: item-scale correlation correcting for overlap

Hi

A lot of papers will have item-scale correlation when trying to determine item score with scale score in order to determine discriminant validity. However, a lot of them will have the words (corrected for overlap) underneath. I'm wondering what's the procedure? For example here: http://medwelljournals.com/fulltext/...i.2009.974.977 "item-scale correlation after correction for overlap"

Just as an extra, it talks about it here:

"Given a set of items to be scored as (perhaps overlapping) clusters and the intercorrelation matrix of the items, find the clusters and then the correlations of each item with each cluster.  Correct for item overlap by replacing the item variance with its average within cluster inter-item correlation." (R: Find item by cluster correlations, corrected for overlap and...)

In R, there is a command called r.cor which, "corrects for the item overlap by subtracting the item variance but then replaces this with the best estimate of common variance, the smc (Guttman's reliability index)."

I'm just unsure how to actually do this in SAS.

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