You can estimate growth curves of the mean number of failures over time. Use these curves to determine whether a trend exists in times between successive failures, and whether the system failures are becoming more frequent, less frequent, or remaining constant.
A reliability engineer assesses the failure rate of a specific air conditioning unit that is used in commercial jet planes. The engineer collects failure data for air conditioning units in 13 airplanes. Each time a unit failed, it was repaired and returned to service.
The engineer wants to determine whether the failure rate is increasing, decreasing, or remaining constant over time. For these data, no air conditioning units were retired from service. All the data are exact failure times.