Psychometrics is a field of study concerned with the theory and technique of psychological measurement. One part of the field is concerned with the objective measurement of skills and knowledge, abilities, attitudes, personality traits, and educational achievement. For example, some psychometric researchers have, thus far, concerned themselves with the construction and validation of assessment instruments such as questionnaires, tests, raters' judgments, and personality tests. Another part of the field is concerned with statistical research bearing on measurement theory (e.g., item response theory; intraclass correlation).