Introduction
Psychological assessment is the systematic collection and interpretation of information about a person's mental functioning. Its aim is to describe the person, support diagnosis, plan treatment and monitor change. The methods include observation, interview, questionnaires, projective techniques, tests of ability and psychophysiological recording. All of them must meet basic standards of quality.
Basic principles of assessment
- Reliability is the consistency of a measure. It is tested as test-retest reliability, internal consistency (Cronbach's alpha) and inter-rater agreement.
- Validity is whether a measure captures what it claims to. Content validity covers the domain, construct validity the underlying concept, and criterion validity the agreement with an external outcome.
- Standardisation means that the procedure, instructions and scoring are the same for everyone. Norms are the scores of a reference population that allow an individual result to be interpreted.
- Ethical principles require informed consent, confidentiality and the use of tests only by trained professionals.
Observation and interview
- Behavioural observation records what a person does in a defined setting. It is useful for children, for people with limited language and for measuring change in behaviour.
- Interviews may be structured (fixed questions), semi-structured (a guide with flexibility) or unstructured. Structured interviews are more reliable, while unstructured interviews give more depth.
- Observation and interview are the first steps of almost every assessment. They also inform the choice of further tests.