Graph of the survival analysis as presented by Nachemson and Peterson (1995). Per definition every patient being progressive is eliminated from the count of the study and therefore has not survived. At the start of the observation period therefore we have 100% of patients in the study and at the end of the observation period there are 30% left (non progressive) in the observation group and 70% left (non progressive) in the patient group treated with a Boston brace (Figure modified according to Nachemson and Peterson 1995). With kind permission of Pflaum, Munich (Weiss HR: Best practice in conservative scoliosis care. 4th edition in press).