Aim

- searching for existing measurement instruments
- choosing the best measurement instrument

Description

The first step in selecting a measurement instrument is to define precisely what your aim of measurement is. Therefore the following issues needs to be considered.

1. Definition of the construct and conceptual model
   The construct of your interest should be defined. For example, when you want to measure physical functioning, you could be interested in what people think they can do, what they actually do (e.g. they may not walk stairs because there is always an elevator to use), or what they can do (e.g. under supervision of a physiotherapist). Although these constructs are closely related, they are not the same. The construct of interest indicates what kind of measurement instrument you should select, e.g. a self-reported or interview-administered questionnaire, a performance-based test, or a clinical measure. It is also important to consider how your construct of interest differs from other closely related constructs. This can help you to further define what your want, but also what you do not want to measure. For example, if you want to measure participation, your need to think about whether participation is the same as social functioning or role functioning, or whether it is something else, and what makes it distinct from these other concepts. Models like the International Classification of Functioning (ICF) (WHO, 2011) or the model by Wilson and Cleary (1995) can be used as a conceptual model to relate the construct in which you are interested in to other related and unrelated constructs.

2. Measurement aim (discrimination, evaluation, prediction)
   A measurement instrument may be applied as a discriminative instrument (to measure differences between people at a single point in time), as an evaluative instrument (to measure changes within people over time), or as a predictive instrument (to classify individuals according to their prognosis). The aim of measurement has consequences for which measurement properties are most important. Validity and interpretability are important for all purposes. When your aim is to discriminate between people, reliability is very important as well. When your aim is evaluation, measurement error and responsiveness are important (see also point 6).

3. Target population
   A measurement instrument should be tailored to the target population (e.g. age, gender, diagnosis, severity of disease, and setting). This is important, because measurement instruments, or items within an instrument may be appropriate for some patients, but not for other patients. Moreover, measurement properties differ between populations.

4. Individual or group level
   Measuring individual patients makes higher demands on reliability. On group level, measurement error is smaller.
   

5. Feasibility
   Take issues into account such as costs, duration, required expertise of the target population, and respondent burden.

6. Measurement properties
   Validity and interpretability are always important. When an instrument is used in a discriminative application, reliability is important, and when an instrument is used in an evaluative application, measurement error and responsiveness are important. When instruments are used to detect very small changes, or on individual level, it is important that the measurement error of the instrument is extremely small. Internal consistency is important for multi-item (sub) scales measuring unidimensional constructs.
   
   We refer to www.cosmin.nl for an overview and definitions of all important measurement properties, and a checklist to assess the methodological quality of studies on measurement properties. This checklist can also be used as a guide for designing a validation study (see guideline Evaluation of measurement properties).

When it is clear what kind of measurement instrument you want to use, there are several ways to find instruments. At www.kmin-vumc.nl a number of links can be found to electronic databases and websites that contain measurement instruments.
When it is unclear which measurement instrument is the best available, you could base your decision on a systematic review on measurement properties. At www.kmin-vumc.nl a list of existing systematic reviews on measurement instruments can be found. When there is no review on instruments for your construct of interest, you may need to perform a search yourself in databases like PubMed. A useful search filter to find studies on measurement properties is developed by Terwee et al. (2009). It could be worthwhile to write a systematic review yourself. There is a high need for high quality systematic reviews of measurement properties. A thorough explanation of how to perform a systematic review on measurement properties of measurement instruments can be found in De Vet et al. 2011. You can also collaborate with the Knowledge Center Measurement Instruments (www.kmin-vumc.nl).

When the best instrument is not available in Dutch, it needs to be translated using a forward and backward method. We refer to a guideline available (in Dutch) at www.kmin-vumc.nl, or the guidelines proposed by Beaton et al. 2000.

 

• Beaton, D. E., C. Bombardier, F. Guillemin, and M. Bosi Ferraz. 2000. Guidelines for the process of cross-cultural adaptation of self-report measures. Spine 25:3186-3191.vragenlijsten/Beaton.pdf
• Van den Brink WP, Mellenbergh GJ. 1998. Testleer en testconstructie. Boom , Amsterdam.
• De Vet, H. C. W., R. W. Ostelo, C. B. Terwee, N. Van der Roer, D. L. Knol, H. Beckerman, M. Boers, and L. M. Bouter. 2006. Minimally important change determined by a visual method integrating an anchor-based and a distribution-based approach. J Clin. Epidemiol.
• De Vet, H. C., C. B. Terwee, L. B. Mokkink, and D. L. Knol. 2011. Measurements in Medicine. A practical guide. Cambridge University Press.
• Fayers, P. M., D. J. Hand, K. Bjordal, and M. Groenvold. 1997. Causal indicators in quality of life research. Quality of Life Research 6:393-406.
• Mokkink, L. B., C. B. Terwee, D. L. Patrick, J. Alonso, P. W. Stratford, D. L. Knol, L. M. Bouter, and H. C. de Vet. 2010. The COSMIN study reached international consensus on taxonomy, terminology, and definitions of measurement properties for health-related patient-reported outcomes. J. Clin. Epidemiol. 63:737-745.
• Sprangers, M. A. and C. E. Schwartz. 1999. Integrating response shift into health-related quality of life research: a theoretical model. Soc. Sci. Med. 48:1507-1515.
• Terwee, C.B., Jansma, E.P., Riphagen, I.I., de Vet, H.C.W. 2009. Development of a methodological PubMed search filter for finding studies on measurement properties of measurement instruments. Qual. Life Res. 18:115-1123 (open access).
• Wilson, I. B. and P. D. Cleary. 1995. Linking clinical variables with health-related quality of life. A conceptual model of patient outcomes. JAMA 273:59-65.
• World Health Organization. 2011. International Classification of Functioning, Disability and Health (ICF). www.who.int/classifications/icf/en

Term	Explanation
Concept	Global definition and demarcation of the subject of measurement.
Construct	A well-defined and precisely demarcated subject of measurement. By psychologists used for unobservable characteristics, such as intelligence, depression, or health-related quality of life.
Conceptual model	Theoretical model of how different constructs within a concept are related, e.g. the Wilson and Cleary* model of health status.
Patient-reported outcomes (PRO)	A measurement of any aspect of a patient’s health status that comes directly from the patient, without interpretation of the patient’s responses by a physician or anyone else.
Non-PRO measurement instruments	All other types of measurement instruments, e.g. clinician-based reports, imaging techniques, biochemical analyses, or performance-based tests.
Health-related quality of life (HRQL)	An individual's perception of how an illness and its treatment affect the physical, mental, and social aspects of his or her life.

V2.0: 27 May 2011 Guideline entirely rewritten and divided in 3 guidelines.
V1.0 1 Jan 2010: Translation into English and updated
V1.2: 2 March 2007,   Point added about questionnaires copyright.
V1.1: 13 February 2006, References updated, text improved in various places, sections adapted.
V1.0: This guideline has been rewritten entirely.

1. Are the concepts that are intend to measure clearly described a priori in the study protocol?
2. Has a systematic literature review been carried out in order to find the most appropriate questionnaire per concept?
3. Is there sufficient evidence to support the selection of the questionnaires used? Have the measurement properties been evaluated?