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Objective: To examine the use of the term ‘metric’ in health and social sciences’ literature, focusing on the interval scale implication of the term in Modern Test Theory (MTT).
Materials and methods: A systematic search and review on MTT studies including ‘metric’ or ‘interval scale’ was performed in the health and social sciences literature. The search was restricted to 2001–2005 and 2011–2015. A Text Mining algorithm was employed to operationalize the eligibility criteria and to explore the uses of ‘metric’. The paradigm of each included article (Rasch Measurement Theory (RMT), Item Response Theory (IRT) or both), as well as its type (Theoretical, Methodological, Teaching, Application, Miscellaneous) were determined. An inductive thematic analysis on the first three types was performed.
Results: 70.6% of the 1337 included articles were allocated to RMT, and 68.4% were application papers. Among the number of uses of ‘metric’, it was predominantly a synonym of ‘scale’; as adjective, it referred to measurement or quantification. Three incompatible themes ‘only RMT/all MTT/no MTT models can provide interval measures’ were identified, but ‘interval scale’ was considerably more mentioned in RMT than in IRT.
Conclusion: ‘Metric’ is used in many different ways, and there is no consensus on which MTT metric has interval scale properties. Nevertheless, when using the term ‘metric’, the authors should specify the level of the metric being used (ordinal, ordered, interval, ratio), and justify why according to them the metric is at that level.
Metric properties of the Spinal Cord Independence Measure - Self Report in a community survey
(2016)
Objective: The Spinal Cord Independence Measure – Self Report (SCIM-SR) is a self-report instrument for assessing functional independence of persons with spinal cord injury. This study examined the internal construct validity and reliability of the SCIM-SR, when administered in a community survey, using the Rasch measurement model.
Methods: Rasch analysis of data from 1,549 individuals with spinal cord injury who completed the SCIM-SR.
Results: In the initial analysis no fit to the Rasch model was achieved. Items were grouped into testlets to accommodate the substantial local dependency. Due to the differential item functioning for lesion level and degree, spinal cord injury-specific sub-group analyses were conducted. Fit to the Rasch model was then achieved for individuals with tetraplegia and complete paraplegia, but not for those with incomplete paraplegia. Comparability of ability estimates across sub-groups was attained by anchoring all sub-groups on a testlet.
Conclusion: The SCIM-SR violates certain assumptions of the Rasch measurement model, as shown by the local dependency and differential item functioning. However, an intermediate solution to achieve fit in 3 out of 4 spinal cord injury sub-groups was found. For the time being, therefore, it advisable to use this approach to compute Rasch-transformed SCIM-SR scores.