[Rasch] R: Construct Validity, Convergence Validity and Discriminant Validity
trevor.bond at jcu.edu.au
Wed Jun 15 10:07:57 EST 2011
Fabio helped by saying : The Rasch modelling framework deals with the “internal” (or structural) construct validity of an instrument: the actual responses to the items of the investigated tool are compared with the responses predicted by a measurement model (the Rasch model) that tells you how the responses to the items should be if the tool satisfied the requirement for fundamental measurement.
Wikipedia gives this as a starting point: In science (e.g. social sciences and psychometrics), construct validity refers to whether a scale measures or correlates with the theorized psychological scientific construct (e.g., "fluid intelligence") that it purports to measure. In other words, it is the extent to which what was to be measured was actually measured. It is related to the theoretical ideas behind the trait under consideration, i.e. the concepts that organize how aspects of personality, intelligence, etc. are viewed. The scale seeks to operationalize the concept, typically measuring several observable phenomena that supposedly reflect the underlying psychological concept. Construct validity is a means of assessing how well this has been accomplished. In lay terms, construct validity answers the question:
"Are we actually measuring (are these means a valid form for measuring) what (the construct) we think we are measuring?"
Then, construct validity is asserted by a conceptual argument – and some of the evidence is statistical. The Rasch evidence can be used to show that empirically, the scale manifests in practice, what the theory being examined requires. So construct validity starts with a theoretical examination of the construct – i.e., substantive theory and some hypotheses about what a scale about that latent trait should exhibit in practice. The Rasch evidence shows the (mis) matches between theory and practice.
i.e., there is no psychometric indicator or set of indicators which prove construct validity. You make a theory based argument and use Rasch etc to help to make your case.
A number of us have written about Rasch indicators and Messick’s view of validity.
Prof. Trevor G Bond
Faculty of Education
Universiti Kebangsaan Malaysia
43600 Bangi, Selangor
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