"Validity … tells us whether the question, item or score measures what it is supposed to measure" (Oppenheim, 1992, p. 144-5).

It is possible for a measure or scale to be reliable but not valid: It may give you similar answers on different occasions, other things being equal (reliability), but it might be doing this because it is consistently measuring something that is NOT the thing you want it to measure.

Reference:

Oppenheim, A. N. (1992) Questionnaire Design, Interviewing and Attitude Measurement: New Edition. London, Pinter.

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