# Reliability Glossary

## Testing and Analysis Terms

**ANOVA**

ANOVA stands for __an__alysis __o__f __va__riance, a method by which
the source of variability is identified. This method is widely used in industry
to help identify the source of potential problems in the production process and
identify whether variation in measured output values is due to variability
between various manufacturing processes, or within them. By varying the factors
in a predetermined pattern and analyzing the output, one can use statistical
techniques to make an accurate assessment as to the cause of variation in a
manufacturing process.

**Competing failure modes**

A model whereby items that fail due to more than one failure mode can be
represented as a series reliability system with each block representing a
failure mode. The failure modes are considered to be "competing" amongst each
other to see which one will cause the item to fail.

**Contour plot**

A graphical representation of the possible solutions to the likelihood ratio
equation. This is employed to determine confidence bounds as well as make comparisons
between two different data sets.

**Degradation analysis**

A technique that uses the performance (degradation) measurements of a product over time to predict the point at which each unit in the sample is expected to fail. This analysis is useful for tests performed on products with very high reliability, where it is not possible to test the units to failure under normal conditions.

**Demonstration testing**

A testing methodology, the goal of which is
to show that a product meets a given reliability target (e.g., that it has a
reliability of at least 90% at 100 hours). The term is often used to refer to
"pass/fail" tests that involve observing the number of units that have failed by
a given time, rather than recording the failure times for each unit (i.e.,
reliability testing).

**Importance measure**

A measure of the relative contribution of a component’s contribution to the
overall system’s reliability. The importance measure of a component is
equivalent to the first partial derivative of the component reliability with
respect to the system reliability.

**Life data analysis**

The statistical analysis of failure and usage data performed in order to mathematically model the reliability and failure characteristics of a
product.

**Monte Carlo simulation**

A method of generating values from a known distribution for the purposes of
experimentation. This is accomplished by generating uniform random variables and
using them in an inverse reliability equation to produce failure times that
would conform to the desired input distribution.

**Nonparametric analysis**

A method of analysis that allows the user to characterize failure data without
assuming an underlying failure distribution. This avoids the potentially large
errors brought about by making incorrect assumptions about the distribution.
However, the confidence bounds associated with nonparametric analysis are
usually much wider than those calculated via parametric analysis. Additionally,
predictions outside the range of the observations are not possible.

**Plotting paper
**

*see Probability plotting paper*

**Probability plot**

A type of plot that linearizes a distribution’s * cdf*, allowing the user
to manually plot failure time vs. estimated unreliability. Provided that the
plotted points fall on a relatively straight line (thus indicating that the
chosen distribution is a good fit), the parameter estimates can be obtained from
scales on the plot. This is a crude, time-consuming method of fitting a
distribution to failure data, but it was practically the only method available
prior to the widespread use of computers.

**Probability plotting paper**

A specially designed type of graph paper that allows the user to plot failure
time vs. unreliability as a linear function. Plotting paper constructions varies
from distribution to distribution. Probability plotting papers that have been
generated by ReliaSoft's software are available on the Web at
http://www.weibull.com/GPaper/index.htm.

**Recurrent event data analysis**

A testing methodology where the observed events are dependent and not identically distributed (such as repairable system data), or where the analyst is interested in modeling the number of occurrences of events over time rather than the length of time prior to the first event
(as in
life data analysis).

**Reliability test design**

The process of designing plans for reliability testing.

**Reliability testing**

Testing units to failure in order to obtain raw failure time data for
life data
analysis.

**Sequential testing**

A testing methodology in which test units are tested consecutively instead of
simultaneously.

**Spares provisioning**

The stocking of spare units or components based on the anticipated number of
failures for a given mission or length of operation.

**SPRT**

SPRT stands for __s__equential __p__robability __r__atio __t__est.
This is a type of accept/reject sequential
testing in which accept/reject boundaries are defined by the user and units
are sequentially tested until either the accept boundary or the reject boundary
have been reached and a decision is made about the suitability of the units.

**Stress-strength interference**

A method by which the probability of failure of an item is calculated by
superimposing the distribution of the item’s strength over the distribution of
the stress it will encounter during normal usage.

**Warranty analysis**

The analysis of warranty and return data for the purpose of determining the
reliability characteristics of a product.

**Zero-failure test**

A "pass/fail" demonstration test,
where a product is shown to meet a specified reliability requirement if all
units survive the test.