Unreliability or unavailability of the component is the unit used by the time independent distribution. It is the probability that the component will not operate properly. This distribution is used when the probability of failure is assumed to remain constant, which means that it does not vary with time. The time independent distribution is used only when there is not enough information about the pattern of the distribution. An unreliability or unavailability of the component corresponding to mission time can then be specified. It is also used to specify steady state unavailability. Care should be taken when analyzing time-specific results.

Because time-independent components are not repairable, maintenance activities are not supported for time-independent distributions. Thus, in the RBD module, the properties on the Periodic page and all three Sparing pages are unavailable when time independent is the distribution.