1 Department of Health Economics and Management, Faculty of Public Health, Medical University of Varna , Varna , Bulgaria.
2 Department of Health Services Research, Faculty of Health, Medicine and Life Sciences, CAPHRI, Maastricht University Medical Center, Maastricht University , Maastricht , Netherlands.
3 Department of Health Services Research, Faculty of Health, Medicine and Life Sciences, CAPHRI, Maastricht University Medical Center, Maastricht University , Maastricht , Netherlands ; Topinstitute Evidence-Based Education Research (TIER), Maastricht University , Maastricht , Netherlands.