Psychometrics is the construction and validation of measurement instruments and assessing if these instruments are reliable and valid forms of measurement. In behavioral medicine, psychometrics is usually concerned with measuring individual’s knowledge, ability, personality, and types of behaviors. Measurement usually takes place in the form of a questionnaire, and questionnaires must be evaluated extensively before being able to state that they have excellent psychometric properties, meaning a scale is both reliable and valid.
A reliable scale consistently measures the same construct. This can occur across testing sessions, individuals, and settings. A valid measure measures what it says it is going to measure. If something is valid, it is always reliable. However, something can be reliable without being valid.