The goal of a prediction is compute the failure rate of a system. Failure rate is the rate of occurrence of failures. This value is normally expressed as failures per million hours, but it can also be expressed as failures per billion hours, in which case the units are referred to as FITs (failures in time). The failure rate is basically the anticipated number of times that the item fails in a specified period of time. For example, if a part has a failure rate of 2 failures per million hours, then it is anticipated that this part will fail twice in a million-hour period.

As described in Prediction Calculation Models, component failure rates are generally calculated based on established reliability prediction standards. MIL-HDBK-217 models assume that failure rate units are failures per million hours, while Telcordia models assume that failure rate units are failures per billion hours. In the System file properties, you can specify whatever failure rate units you prefer.

When specifying data, you indicate whether a failure rate is calculated or specified. A calculated failure rate is based on the parameters required by the selected calculation model. Generally, parameters include part temperature, environment, and stress. Summing the failure rates for the parts within an assembly provides the failure rate for that assembly. Summing the failure rates for assemblies provides the failure rate for the system.

Failure rates for many basic parts are well below one failure per million hours, which makes these values seem insignificant. However, if you have hundreds of parts in the system and thousands of systems operating in the field, these failure rates quickly add up.