The prior definition of “unknown” was: Identifies that the method used to obtain information about the subject is unknown.   The new definition of “unknown” is: A proper value is applicable, but not known.   The new definition clarifies that a value is applicable but is not known. Ideally, the method used to obtain any patient care data should be known. However, this is not always the case. For example, method may not have been required, there may not have been a field to record method, or the method by which the data was acquired may not be available from an external system. Where possible, “other-specify” can be used. However, in some cases, “Unknown” may be the most accurate/truthful representation of the data.