Reliability and validity are not simple have/don't have conditions, but instead an equation in which it is desirable, but often impossible to maximise both. This is reflected in Bachman and Palmer's concept of usefulness in which a test's usefulness is defined as:Usefulness = Reliability + Construct Validity + Authenticity +
Interactiveness + Impact + Practicality (1996:18)
While a full discussion of this is beyond the scope of this paper, the concept of usefulness and the tension between reliability and validity must be considered when looking at testing decisions.