A Weibull failure distribution is often used to characterize failure times in reliability models. It has two parameters: a Slope and a Mean Lifetime. The slope is dimensionless and the mean lifetime has dimensions of time. The slope controls the shape of the distribution. A slope of 1 produces an exponential curve; a slope of 3.3 produces a normal distribution; slopes between 1 and 3.3 produce normal-like curves with a trailing tail (see below); slopes greater than 3.3 produce normal-like curves with a leading tail.
The curve below shows a Weibull distribution that has been delayed by 10 years:
Note: The Slope and Mean
Lifetime should not be specified as functions of time. If they are, only
the initial values will be used.