Method III, also referred to as the black box or parts count method integrated with field data, is used when actual field or life data is known and you want to take this data into account during failure rate calculations. It modifies the results for steady state failure rates based on a weighted average for the field data. When the number of total operating hours for the device or unit is large, failure rate predictions are heavily influenced by the field data. When the number of total operating hours for the device or unit is small, failure rate predictions may be only slightly influenced by the field data.

The various scenarios available with Method III include (a), (b), and (c). The following table describes each of these scenarios.