1. BACKGROUND Cost-effectiveness of Psychotherapy for Cluster C Personality Disorders and the Value of Information and Implementation Djøra I. Soeteman 1,2, MA, Jan J. van Busschbach 1,2, PhD, Roel Verheul 1, PhD, Jane J. Kim 3, PhD 1 Viersprong Institute for Studies on Personality Disorders, Halsteren, 2 Dept. of Medical Psychology and Psychotherapy, Erasmus MC, Rotterdam, The Netherlands, 3 Center for Health Decision Science, Harvard School of Public Health, Boston, MA METHODS OBJECTIVES MODEL STRUCTURE  There are three questions which any health care system needs to address in order to improve the efficiency of health- care provision: 1). Which treatments are cost-effective and should be adopted? (reimbursement decision); 2). Is it worthwhile to conduct additional research (research decision); 3). Is it cost-effective to implement treatments into clinical practice (implementation decision). REIMBURSEMENT DECISION We acknowledge the contributions of the participating specialist centers of psychotherapy in The Netherlands: Center of Psychotherapy De Gelderse Roos, Lunteren; Medical Center Zaandam; Altrecht, Utrecht; Center of Psychotherapy Arkin, Amsterdam; GGZWNB, Bergen op Zoom; Center of Psychotherapy De Viersprong, Halsteren. IMPLEMENTATION DECISION  To evaluate the cost-effectiveness of five dosages of psychotherapy in treating avoidant, dependent, and obsessive-compulsive (i.e., cluster C) personality disorders (PD).  To assess the societal value of additional research and to identify the model parameters for which reduction of uncertainty would be most valuable.  To estimate the societal benefit associated with changing the capacity of and adherence to cost-effective treatment strategies.  Using patient-level primary data (n=466) from the largest existing clinical trial of psychotherapy for PD (i.e., SCEPTRE), we developed a probabilistic Markov cohort model.  This study compares dosage, specified by a combination of treatment setting (outpatient, day hospital, and inpatient) and duration (short-term or up to six months versus long-term or more than six months).  The analytic time horizon was five years, modeled as six- month time cycles.  Value of information and value of implementation analyses were conducted to evaluate the parameters that were responsible for most of the decision uncertainty, and to calculate the expected values of perfect information (EVPI) and perfect implementation (EVPIM). RESEARCH DECISION CONCLUSIONS  Based on a threshold value of €40,000 per QALY, short-term inpatient psychotherapy was identified as the most cost-effective treatment option.  The societal value of additional research on treatments for cluster C PD is substantial, especially when prioritizing information on treatment costs and transition probabilities.  Implementing this cost-effective treatment strategy into clinical practice is likely to improve the efficiency of health-care provision in The Netherlands.  The underlying clinical process driving the model and by which the health states are defined is ‘clinically significant change’, based on a statistical approach to defining meaningful change in psychotherapy research.  Transition probabilities were adjusted for selection bias with the multiple propensity score method.  The population expected value of perfect information (EVPI) is €587 million assuming a five-year lifetime for psychotherapy (€5,884 for individual patients).  The expected value of perfect implementation (EVPIM), based on the current level of information, was €4,381 million for the eligible population (€43,915 for individual patients).  The population expected value of partial perfect information (EVPPI) for the four groups of parameters at a cost-effectiveness threshold of €40,000 per QALY and a five-year lifetime for psychotherapy shows relatively high values associated with transition probabilities and treatment costs. REIMBURSEMENT DECISION  The cost-effectiveness acceptability frontier (CEAF) displays the switch point, at which there is a change in the optimal option from short-term day hospital psychotherapy to short-term inpatient psychotherapy (€16,570 per QALY).  The table summarizes the model projections of costs and effects of the five alternative treatment dosages of psychotherapy for cluster C PD, and highlights the most cost-effective choice (i.e., the option with the highest incremental cost-effectiveness ratio (ICER) below the threshold, or the option that maximizes net monetary benefit) at a willingness-to-pay threshold of € 40,000 per QALY.