What is Psychometrics?
Psychometrics is the science of measuring psychological attributes — ability, personality, knowledge, attitude — that cannot be observed directly. It covers how questions are written and calibrated, how responses become scores, how much error surrounds a score, and what evidence is required before a score may support a decision about a person.
The two halves of the field
One half is about instruments: item writing, scaling, calibration, reliability, dimensionality, fairness analysis. The other half is about inference: what a score licenses you to conclude, and for which use.
Most commercial disputes about assessment are really disputes about the second half. An instrument can be technically sound and still be used to support a conclusion it was never validated for, which is why validity is defined in relation to a purpose rather than as a property of the test.
What a psychometrically literate buyer asks
What is the construct, stated in one sentence. What reliability figure is claimed, of which kind, on how many items and how many people. What the norm group is. What the standard error of measurement is in the units of the reported scale. What validity evidence exists for this use, not for the instrument in general.
Five questions, all answerable in a sentence each by a vendor who has the evidence, and a documented refusal by one who does not.
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