PMS (post-marketing surveillance), the practice of monitoring the safety of a drug or medical device after it has been placed on the market, is a vital part of the science of pharmacovigilance. Since drug and medical device approvals are based on clinical trials involving a relatively small number of individuals selected for this purpose-meaning they typically do not have other diseases that may be present in the general population-post market surveillance can refine or confirm or deny the safety of a drug or medical device further after it has been used in the general population by a large number of individuals with a variety of diseases. A variety of approaches are used to monitor the post-market safety of drugs and devices, including spontaneous reporting databases, monitoring of prescription events, electronic health records, patient registries, and linkage of health databases to identify potential safety concerns in a process known as data mining.