108 | Chapter 4 Control Preference Scale The Control Preference Scale (CPS) is a validated questionnaire comprising 5 items and assesses patients’ desire for involvement in decision making. The items are: I prefer (1) to make the decision alone, (2) to make the decision alone, after considering the physicians’ opinion, (3) to make the decision together with the physician, (4) the physician to make the decision after considering my opinion, (5) the physician to make the decision alone9. Knowledge Test A study-specific knowledge test was constructed to assess patients’ knowledge after counseling. It used 20 statements on the different treatment options, which could be rated as “true”, “not true” or “do not know”. The knowledge test included eight questions on TL, five on RT and seven on CRT. Internal consistency was assessed using Cronbach’s alpha, to ensure that the questionnaires’ items consistently measured the same underlying construct. The number of correctly answered questions was divided by the total number and multiplied by 100% to obtain a percentage of correctly answered questions. The same scoring was done for the subscores per treatment option and treatment group. Statistical analyses All statistical analyses were performed with IBM SPSS Statistics for Windows, Version 27 (2020; Armonk, NY: IBM corp.). Descriptive statistics were used to summarize the samples’ characteristics and describe the levels of DC. Spearman correlations were performed to explore which variables are associated with DC. Testing for differences over time (T1 and T2) were done with the appropriate test depending on type and distribution of the data. A Bland-Altman analysis was employed to assess the agreement and potential differences in responses to Shared Decision-Making (SDM) questionnaires between physicians and patients. Ethics This study was conducted according to the principles of the Declaration of Helsinki30. The study received approval from the Ethical Review Committee of the Netherlands Cancer Institute under registration number IRBd22-023, and each local medical ethics review board at participating centers also granted their approval.
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