AssessmentThe 9-item Shared Decision Making Questionnaire (SDM-Q-9). Development and psychometric properties in a primary care sample
Introduction
The Shared Decision Making model (SDM) is a clinical decision-making model that ensures that health care professionals do not make decisions solely on the basis of knowledge, experience, and the latest scientific evidence, but that they also inform patients broadly and let them take part in all important aspects of the medical decision [1]. In the literature, several definitions of SDM exist [2], [3]. In this study, SDM is defined as an interactive process in which both parties (patient and physician) are equally and actively involved and share information in order to reach an agreement, for which they are jointly responsible [4]. Although the practical applications of SDM have clearly gained in importance in the past decade, the measurement of its constructs (e.g. preferences for information and involvement, decisional conflict) remains challenging [5], [6], [7], [8], [9]. Several studies focussed on the administration of decision aids, but as a recent analysis pointed out, these approaches are not always theoretically underpinned [10]. In contrast to its outcomes, the process of SDM is rather underresearched and has not received much attention in the past.
The German Ministry of Health funded a research consortium on SDM from 2001 until 2005 with a distinct emphasis on the measurement of constructs that are related to SDM. After reviewing existing instruments and empirical findings on their psychometric properties, the methods group of the consortium reached the conclusion that no theory-driven, psychometrically sound self-assessment tool was available to measure the process of SDM from the patient's perspective [11]. In order to close this gap, the methods group developed such an instrument, preliminarily named SDM-Q. As a first step towards a theory-driven tool, the underlying concept was clarified by defining theoretical key features and nine practical steps [12], which are displayed in Fig. 1. The rigorous development process and the first endeavour to establish reliability and validity of the SDM-Q applying item response theory (Rasch model) led to several conclusions [12]: first, the instrument proved to be inconsistent with the probability-based underlying theory. Our assumption, that the comparison of two subjects regarding the measured construct (SDM) is independent of which items may be used within the defined set of items, could not be satisfied. Several analyzed items showed non-uniform characteristics through indicating varying distances between thresholds on the applied four-step rating scale. Second, only five of the nine theoretically defined steps of the SDM process (cf. Fig. 1) could be measured sufficiently. Third, strong ceiling effects on the applied four-step rating scales were observed, with most patients marking the option corresponding to the highest degree of experienced SDM in each item. Fourth, relevant differences were found in the use of the instrument between patients with different diseases (differential item functioning). The comparison of samples covering the medical fields of depression, urology, anaesthesia, gynaecology, and general practice revealed that some items (partly covering steps 4, 5, 7, 8, and 9) were interpreted variably across conditions.
The present contribution reports on the second test development phase addressing these problems. For reasons of clarity, the revised instrument was named SDM-Q-9. The objective of this study was to develop and test a valid, reliable and brief instrument to measure the process of SDM in clinical encounters from the patient's perspective.
Section snippets
Revision of the instrument
The original instrument (SDM-Q) underwent a major revision addressing the findings of the first development phase [12]. First, the applied test theory was changed. Initially, we assumed that each item represented the whole construct that was intended to be measured (SDM process). Thus, we used item response theory (Rasch model) [13]. However, after being confronted with the non-conformity of the applied model with empirical data, we revised our assumptions and decided to use classical test
Sample characteristics
Of the 2450 persons who responded to the questionnaire, 2351 provided analyzable data. The above-described random split of the sample into two halves led to a development sample of 1188 persons and to a test sample of 1163 persons. Demographic, clinical, and decision-related characteristics are reported in Table 1.
Slightly more women than men were included in the total analysis sample. 62.1% of the sample was older than 60 years and only a minority (5.0%) had a completed secondary or higher
Discussion
This study shows that the second test development phase was able to deal with the problems of the first version of the Shared Decision Making Questionnaire (SDM-Q) [12] by applying an elaborate multistep approach. Classical test theory was applied, a new core set of 26 items was created and the response format was changed. By splitting the sample into a development and test sample, cross-validation of the results became possible. The revised instrument (SDM-Q-9) consists of 9 items, one for
Role of funding
The sponsors were not involved in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.
Conflict of interest
None.
Acknowledgements
The project in the context of which the present study was performed was conducted in cooperation with the Gesundes Kinzigtal GmbH and the Department of Medical Sociology, University of Freiburg and was funded by the Health Insurance Company AOK Baden-Württemberg and the Health Insurance Company LKK Baden-Württemberg. We would like to thank Dominick Frosch and Jana Hoffmann for their collaboration in the translation process. We also thank Sarah Mannion for copyediting the manuscript.
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