Clinimetrics: how to define the quality of a measurement instrument?
The quality of an outcome measurement instrument can be assessed by evaluating its measurement properties. For (patient-reported) questionnaires, the most important measurement property is the content validity.
What is clinimetrics and why is it important?
Clinimetrics is the science revolving around clinical measurements. This field of study is important in order to ensure the quality of measurement instruments. For this reason, available measurement instruments are subjected to a rigorous clinimetric appraisal. In clinical studies, measurements instrument are used to evaluate a specific situation or disease within patient populations. The results acquired with these instruments must be trustworthy and representative of the true situation.
Content validity
Content validity is defined as “the degree to which the content of an instrument is an adequate reflection of the construct to be measured” [1] . In other words, is the content of the questionnaire relevant, all-inclusive, and understandable for the construct (i.e. concept that the questionnaire aims to measure), and target population? It is not easy to evaluate the content validity of a questionnaire, especially since many questionnaires aim to measure complex and unobservable concepts, for example, quality of life. The difficulty here can be found in the definition and operationalization of the construct. For example, how is “quality of life” being defined by and which concepts are considered important for the target population. Defining and understanding such difficult concepts is the first step within a content validity study.
The content validity of patient-reported outcome measurement instruments can be assessed by conducting qualitative studies, for example using focus groups or interviews, with the target population.
Reliability
Measurement instrument with a high reliability are measurement instruments that are free from measurement error. When a patient, who’s disease has not changed, completes a questionnaire twice, you expect to get the exact same results. If this is not the case, the questionnaire possesses a measurement error. Measurement error is the clinically most important aspect of reliability, and is defined as “the systemic and random error of a patient’s score that is not attributed to true changes in the construct to be measured [2] ”. It is important to consider that all measurement instruments, even of the highest quality, possess a measurement error. With that being said, the measurement error should be small enough to allow relevant changes to be measured.
Responsiveness
Besides the ability to obtain the same results after repeated measurements of a stable person, it is also very important to detect changes in scores when persons have changed. The responsiveness is defined as: “the ability of an instrument to detect change over time in the construct to be measured”. It refers to the validity of the change score of an instrument.
Mokkink, L. B., Terwee, C. B., Patrick, D. L., Alonso, J., Stratford, P. W., Knol, D. L., Bouter, L. M., & de Vet, H. C. (2010). The COSMIN study reached international consensus on taxonomy, terminology, and definitions of measurement properties for health-related patient-reported outcomes. Journal of Clinical Epidemiology, 63(7), 737–745.
Mokkink, L. B., Terwee, C. B., Patrick, D. L., Alonso, J., Stratford, P. W., Knol, D. L., Bouter, L. M., & de Vet, H. C. (2010). The COSMIN study reached international consensus on taxonomy, terminology, and definitions of measurement properties for health-related patient-reported outcomes. Journal of Clinical Epidemiology, 63(7), 737–745.
