Weibull Distribution

In probability theory and statistics, the Weibull distribution (named after Waloddi Weibull) is a continuous probability distribution with the probability density function
f(x) = (k/\lambda) (x/\lambda)^{(k-1)} e^{-(x/\lambda)^k} \qquad \mbox{for } x>0
where k >0 is the shape parameter and λ > 0 is the scale parameter of the distribution. The cumulative density function is defined as
F(x) = 1- e^{-(x/\lambda)^k} \qquad \mbox{for } x>0
The Exponential distribution (when k = 1) and Rayleigh distribution (when k = 2) are two special cases of the Weibull distribution. Weibull distributions are often used to model the time until a given technical device fails. If the failure rate of the device decreases over time, one chooses k < 1 (resulting in a decreasing density f). If the failure rate of the device is constant over time, one chooses k = 1, again resulting in a decreasing function f. If the failure rate of the device increases over time, one chooses k > 1 and obtains a density f which increases towards a maximum and then decreases forever. Manufacturers will often supply the shape and scale parameters for the lifetime distribution of a particular device. The Weibull distribution can also be used to model the distribution of wind speeds at a given location on Earth. Again, every location is characterized by a particular shape and scale parameter. The expected value and standard deviation of a Weibull random variable can be expressed in terms of the gamma function:
E(X) = λ Γ((k + 1) / k) and
var(X) = λ2+ 2) / k) - Γ2((k + 1) / k)

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