Sheryl Lee appeared in a film as a teacher.
Mean time between failures -LRB- MTBF -RRB- is the predicted elapsed time between inherent failures of a system during operation .. MTBF can be calculated as the arithmetic mean -LRB- average -RRB- time between failures of a system .. arithmetic mean. arithmetic mean. The term is used in both plant and equipment maintenance contexts .. The definition of MTBF depends on the definition of what is considered a system failure .. failure. failure. For complex , repairable systems , failures are considered to be those out of design conditions which place the system out of service and into a state for repair .. repairable. repairable. Failures which occur that can be left or maintained in an unrepaired condition , and do not place the system out of service , are not considered failures under this definition .. In addition , units that are taken down for routine scheduled maintenance or inventory control are not considered within the definition of failure .. failure. failure