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ITEM Toolkit Manual

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Understanding Analysis Results<br />

Chapter 7 RBD 169<br />

Unavailability Q<br />

Unavailability Q represents the probability that the component or system is unavailable at any given time. “Q” equals<br />

the probability that the system is unavailable.<br />

Failure Frequency W<br />

Failure Frequency W, or unconditional failure intensity, is the probability that the system or component fails per unit<br />

time, given that it was working correctly at time zero. “W” is equal to the number of expected system failures.<br />

CFI<br />

Conditional Failure Intensity. This is the probability of failure per unit time, given that the component was “working-asdesigned”<br />

at time zero and is working at time t.<br />

Expected Failure<br />

Expected Failure is the number of times the system is expected to fail over a specified period of time (lifetime).<br />

Unreliability<br />

Unreliability represents the probability of one or more system failures over a specified period of time. The number of<br />

expected system failures (W) provides a good approximation for system unreliability for cases where W

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