Behavioural and Cognitive Psychotherapy, 2001, 29, 221–233 Cambridge University Press. Printed in the United Kingdom AN INVESTIGATION INTO THE PSYCHOMETRIC PROPERTIES OF THE COGNITIVE THERAPY SCALE FOR PSYCHOSIS (CTS-Psy) Gillian Haddock, Sheila Devane, Tim Bradshaw, John McGovern, Nicholas Tarrier, Peter Kinderman, Ian Baguley, Stuart Lancashire and Neil Harris University of Manchester, U.K. Abstract. Recent research suggests that cognitive-behaviour therapy (CBT) can significantly improve outcomes for patients with severe mental health problems. However, there are no tools specifically designed to assess competence in delivering CBT to psychotic patients. This study investigates the psychometric properties of the Cognitive Therapy Scale for Psychosis (the CTS-Psy) for assessing the quality of CBT with psychotic patients. Inter-rater reliability of trained raters using the CTS-Psy was investigated using taped therapy of trainees engaged in a CBT oriented psychosis training course. Validity was investigated in relation to examining the degree to which the scale could be used to assess a range of therapist ability and patient severity and by assessing the degree to which the CTS-Psy could pick up changes in skill acquisition during the training course over a 9-month period. The CTS-Psy demonstrated excellent inter-rater reliability and good validity in relation to it being able to rate all standards of therapy and all types of patient sessions in the sample studied. In addition, the scale was sensitive to changes in clinical skills during a training course and could discriminate between those who had received training and those who had not. Keywords: CBT, psychosis, cognitive therapy scale-psychosis (CTS-Psy), validity, reliability. Introduction For many years the main treatment for patients experiencing severe and enduring mental health problems such as schizophrenia has been neuroleptic medication. More recently, research has demonstrated that the effectiveness of medication can be enhanced with psy- chosocial treatments such as family interventions (Mari & Streiner, 1994) or individual cognitive-behavioural therapy, in terms of improving relapse rates, reducing the severity of hallucinations and delusions and improving carer functioning (Haddock et al., 1998). How- ever, despite these research findings, the availability of training for mental health profes- sionals in this area is limited and the number of therapists who are completely trained is small. This has resulted in very few psychotic patients receiving any sort of CBT in routine Reprint requests to Gillian Haddock, Department of Clinical Psychology, University of Manchester, Tameside General Hospital, Fountain Street, Ashton under Lyne OL6 9RW, U.K.  2001 British Association for Behavioural and Cognitive Psychotherapies