It is sometimes said that reliability field data is the “real reliability data” because they reflect actual reliability performance of a product or system. Reliability field data are
obtained, most commonly, from warranty returns (combined with production/sales records to provide information on units that were not returned) and maintenance databases. For some products (e.g., medical devices), careful field tracking is done, providing detailed information about all units deployed into the field. Reliability field data are almost always multiply censored because many units had not failed
at the time the data were analyzed. In addition to failure times, sometimes failure mode information is also available for units that have failed. Other complications like truncation also arise in some field reliability data sets.