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st: Intraclass Correlation for an Independent Variable?

From   Lloyd Dumont <>
Subject   st: Intraclass Correlation for an Independent Variable?
Date   Wed, 11 Aug 2010 13:36:01 -0700 (PDT)

Hello.  I have gotten a confusing note from a reviewer, confusing in part because I think he/she is using the wrong terminology.

In a nutshell…

I am running a cross-sectional time series model (on a dataset of plant-months) of a continuous variable Y measured at the plant level where the focal, continuous independent variable is X.  X is also at the plant-level, but is time-constant.  X is formed by taking the mean level of X from all of the individual workers in each plant.

The reviewer suggests that I include some measure of within versus between plant variance in X.  This would bolster the case that there is a genuine difference across plants rather than average differences simply reflecting noise.  He/she then goes on to suggest I do this by reporting intraclass correlations (ICCs).

As far as I understand it, ICCs (or rho) describe the variance in Y, not X.  If I am right, then the reviewer did not mean ICC.

Rather, he/she wants me to go back to the dataset of individuals (not plant-months) and wants me to run some simple analysis to show that there is clustering of values for X by clinic.

Am I correctly understanding what the reviewer is asking for?  And, if so, what is the simplest way to demonstrate the sort of dependence he/she is hoping to see?

Thank you for your help.  Lloyd


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