# Reliability Glossary

## Lifetime Distribution Terms

**Exponential distribution**

A lifetime statistical distribution that assumes a constant failure rate for the
product being modeled.

**Failure distribution**

A mathematical model that describes the probability of failures occurring over
time. Also known as the probability density function (*pdf*),
this function is integrated to obtain the probability that the failure time
takes a value in a given time interval. This function is the basis for other
important reliability functions, including the reliability function, the failure
rate function, and the mean life.

**Gaussian distribution**

*see Normal distribution*

**Generalized gamma distribution**

While not as frequently used for modeling life data as other life distributions,
the generalized gamma distribution does have the ability to mimic the attributes
of other distributions such as the Weibull or lognormal, based on the values of
the distribution's parameters. While the generalized gamma distribution is not
often used to model life data by itself, its ability to behave like other more
commonly-used life distributions is sometimes used to determine which of those
life distributions should be used to model a particular set of data.

**Life distribution**

*
see Failure distribution*

**Lognormal distribution**

A lifetime statistical distribution that is often used to model products in
which physical fatigue is the prominent contributor to the primary failure mode.

**Mixed Weibull distribution**

A variation of the Weibull distribution used to model
data with distinct subpopulations that may represent different failure
characteristics over the lifetime of a product. Each subpopulation has separate
Weibull parameters calculated, and the results are combined in a mixed Weibull
distribution to represent all of the subpopulations in one function.

**Normal distribution
**
A common lifetime statistical distribution that was developed by mathematician
C. F. Gauss. The distribution is a continuous, bell-shaped distribution that is
symmetric about its mean and can take on values from negative infinity to
positive infinity.

**Probability density function ( pdf)**

A mathematical model that describes the probability of events occurring over time. This function is integrated to obtain the probability that the event time takes a value in a given time interval. In life data analysis, the event in question is a failure, and the

**Weibull distribution**

A statistical distribution frequently used in life data analysis. Developed by
Swedish mathematician Waloddi Weibull, this distribution is widely used due to
its versatility and the fact that the Weibull * pdf* can
assume different shapes based on the parameter values.