Basic reliability cookbook
This tutorial
describes how to calculate the reliability of elements combined in
several typical ways. Note
that the terminology used is intuitive and is not intended to be
mathematically precise.
Topics 
Failure Rate
Failure rate is
defined as failures per unit time.
Typical failure rates are expressed as failures per billion (109)
hours (called the FIT rate) or failures per million (106)
hours. A critical factor in
translating a failure rate to the equivalent reliability is knowing the
corresponding unit time.
The reliability of an
element is equivalent to the probability that it survives a period of
time without failing. The
probability of survival PS
for a time period T is e-λT,
where
λ is the
failure rate of the element (and, for the purposes of this tutorial, has
a uniform failure distribution)
For example, an
element has a failure rate of 30 failures per billion hours (30 FITs).
The probability of survival to a billion hours is e(-30*1)
= 9.35762*10-14.
The probability of survival to a million hours is e(-30*0.001)
= 0.97044.
Failure Rate | Serial Reliability | Parallel Reliability | K of N Reliability | Standby Reliability | Duty Cycle Reliability