sentences:  A preference based PRO has an algorithm attached to the PRO instrument which can 'weigh' the outcomes reported by patients according to the preferences for health outcomes of a group of individuals such as the general public or of patient groups.The purpose of this 'weighing' is to make sure that elements of health that are very important receive larger weight when computing sum scores.For example, individuals may consider problems with their mood to be more important than limitations in usual activities.