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Observational Studies

6. Measurement Validity

Measurement validity begins with an assessment of how the target of the measurement is conceptualized. One problem can be "over-coverage." Another problem can be "under-coverage."

In the case of IQ, if the concept of general cognitive ability does not include a full range of cognitive skills (e.g., musical), there is under-coverage. If the concept of cognitive ability includes attributes that are really culturally based (e.g., vocabulary), there is over-coverage.

In the case study by Rush and his colleagues (2008), the questionnaire items seem intuitively sensible, but no definition of the need for treatment is provided. Consequently, both under-coverage and over-coverage are possible.  For example, all but one of the questionnaire items provide a reference time period (e.g., 90 days). There are good reasons for this from the point of view of item construction. One needs to specify a suitable interval for recall. But are there people in need who will be missed without a longer recall interval? And are there features of need that are not addressed with items (a)-(e), or items that are do not really reflect what might be meant by "need?" If yes, the measure is likely to be systematically inaccurate. Some refer to this as measurement bias.
Accounts on the OBSSR e-Learning site enable you to save notes as you read the contents of the site.  Notes are a way for you to save a spot on the site with your own comments and title applied to it.  Think of it as putting a sticky note paper in a book to remember a place and leave a thought or two of your own for later reference.