Re: Question

Posted by Andy W on May 06, 2014; 12:18pm
URL: http://spssx-discussion.165.s1.nabble.com/Question-tp5725795p5725809.html

From a measurement perspective simply summing the items (ignoring the final linear transformation into Z-scores based on the summed items) presumes a parallel measurement model with equal error variances for each instrument. Z-scoring each item individually and then summing them presumes a tau-equivalent model, where the error variances differ between items.

On its face if the instruments have very different variances it would suggest the parallel measurement model is unlikely (unless you have some other insider info.). Typically you would have prior knowledge if the parallel measurement model (or tau equivalent) is reasonable anyway for your given purposes.

It may be the case that neither procedure is appropriate, that is the motivation for confirmatory factor analysis essentially.