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With physical measurements, it is often possible for the same observer (or different observers) to make repeated measurements in quick succession.When there is a subjective element in the measurement the observer can be blinded from their first measurement, and different observers can make simultaneous measurements.With questionnaires, either self administered or recorded by an observer, repeat measurements need to be far enough apart in time for the earlier responses to be forgotten, yet not so far apart that the underlying quantity being measured might have changed.
Our criterion becomes agreement with another indirect measurement.
Cardiac stroke volume does at least have an objective reality; a physical quantity of blood is pumped out of the heart when it beats.
Anxiety and depression do not have a physical reality but are useful artificial constructs.
On the other hand, if the items are too similar, some may be redundant.
Highly correlated items in a scale may make the scale over- long and may lead to some aspects being overemphasised, impairing the content validity.