The RAREEVENTS procedure produces control charts for rare events. A rare event is one that occurs infrequently, with a low probability. A rare events chart is better suited than traditional control charts to detecting changes in the frequency of low-probability events.
The data that are plotted in a rare events chart represent the times between successive events. Usually these are adverse events that represent unwanted outcomes in a process, such as an incorrectly recorded bank deposit, a patient falling in a hospital, or a chemical spill. Rare events charts have gained acceptance in health care quality improvement applications because of their ease of use and suitability to processes that have low defect rates (Benneyan 1999).