# Reliability Glossary

## System Reliability Terms

**Block diagram**

A diagram that
represents how the components, represented by "blocks," are arranged and
related reliability-wise in a larger system. This is often but not
necessarily the same as the way that the components are physically
related. This is also called a reliability block diagram or RBD.

**Complex system**

A block diagram that cannot be reduced to
series and/or parallel systems.

**Decomposition method**

A method for determining the
reliability of complex systems. The decomposition method is an
application of the law of total probability, which involves choosing a
"key" component and then calculating the reliability of the system
twice: once as if the key component failed and once as if the key
component succeeded. These two probabilities are then combined to obtain
the reliability of the system, since at any given time the key component
will be failed or operating.

**Event space method**

A method for determining the reliability
of complex systems. With the event space method, all mutually exclusive
events are determined. The reliability of the system is simply the
probability of the union of all mutually exclusive events that yield a
system success (the unreliability is the probability of the union of all
mutually exclusive events that yield a system failure).

**Path-tracing method**

A method for determining the
reliability of complex systems. With this method, every path from a
starting point to an ending point is considered. Since system success
involves having at least one path available from one end of the
reliability block diagram to the other, as long as at least one path
is available, the system has not failed. The reliability of the system
is simply the probability of the union of these paths.

**Reliability block diagram***see
Block diagram*

**System reliability**

The reliability of an entire system, as
opposed to the reliability of its components. The system reliability is
defined by the reliability of the components as well as the way the
components are arranged reliability-wise.