Causal Variables, Indicator Variables and Measurement Scales: An Example from Quality of Life

Abstract

Summary. There is extensive literature on the development and validation of multi-item measurement scales. Much of this is based on principles derived from psychometric theory and assumes that the individual items form parallel tests, so that simple weighted or unweighted summation is an appropriate method of aggregation. More recent work either continues to promulgate these methods or places emphasis on modern techniques centred on item response theory. In fact, however, clinical measuring instruments often have different underlying principles, so adopting such approaches is inappropriate. We illustrate, using health-related quality of life, that clinimetric and psychometric ideas need to be combined to yield a suitable measuring instrument. We note the fundamental distinction between indicator and causal variables and propose that this distinction suf®ces to explain fully the need for both clinimetric and psychometric techniques, and identi®es their respective roles in scale development, validation and scoring.

Pediatric post-thrombotic syndrome in children: Toward the development of a new diagnostic and evaluative measurement tool

“…Therefore, PTS was considered to be defined precisely by the items (signs and symptoms) that are chosen to measure 25). This conceptualization of the nature of the construct is one of the features of the formative model, and is consistent with the theory underlying the measurement of clinical phenomena (26). Moreover, under the formative model, the items that define PTS are not expected to be interchangeable, but rather each item represents a different facet or aspect of PTS.…”

“…A systematic review of PTS in the pediatric population reported an overall frequency of 26% [95% Confidence Interval (CI): [23][24][25][26][27][28] and a frequency of 17% [95% CI: [14][15][16][17][18][19][20] among prospective studies (9), including both UE and LE venous territories.…”

A Few Thoughts on Measurement Issues

“…The scoring rubric for these "clinimetric" measures typically involves aggregation of item scores by summing or taking the mean of the item set. This scoring approach is based on the assumption that each item is equally important in explaining health status (Fayers & Hand, 2002). The questionable soundness of this assumption is easily demonstrated; for example, when an item assessing the occurrence of chest pain is weighted equally with an item assessing the occurrence of fatigue on a health status symptom measure, or when an insomnia item and a suicidal ideation item are given equal weight on a mental health status measure.…”

ACCF/AHA 2010 Position Statement on Composite Measures for Healthcare Performance Assessment

“…20 This concept attempts to more specifically serve the aim of clinicians, "which is to choose and emphasize suitably the most important attributes to be included in the index, using multiple items which are not expected to be homogeneous because they indicate different aspects of a complex clinical phenomenon" (p 234). 21 The selection and weighting of items is based on deliberate judgment and is considered successful if the behavior of the composite score is consistent with the developer's intentions.…”