If the items to be "summed" or combined to create an overall index are collectively the underlying construct (i.e. I am trying to measure compliance to an intervention which has different components), wouldn't combining all the components for IRT violate the assumption of unidimensionality since they represent the intervention itself? If so, do you have a reference for this?
Are there any good references that say if Likert scores are summed into a scale and the Cronbach's alpha is relatively high (~0.8), this is appropriate to use as an index?
Are there any other tests of validity I should do other than Cronbach's alpha if all I have done is sum the responses?
Thanks in advance, any advice is appreciated!