distributional (version 0.6.0)

dist_weibull: The Weibull distribution

Description

Generalization of the gamma distribution. Often used in survival and time-to-event analyses.

Usage

dist_weibull(shape, scale)

Arguments

shape, scale

shape and scale parameters, the latter defaulting to 1.

Details

We recommend reading this documentation on pkgdown which renders math nicely. https://pkg.mitchelloharawild.com/distributional/reference/dist_weibull.html

In the following, let \(X\) be a Weibull random variable with shape parameter shape = \(k\) and scale parameter scale = \(\lambda\).

Support: \([0, \infty)\)

Mean:

$$ E(X) = \lambda \Gamma\left(1 + \frac{1}{k}\right) $$

where \(\Gamma\) is the gamma function.

Variance:

$$ \text{Var}(X) = \lambda^2 \left[\Gamma\left(1 + \frac{2}{k}\right) - \left(\Gamma\left(1 + \frac{1}{k}\right)\right)^2\right] $$

Probability density function (p.d.f):

$$ f(x) = \frac{k}{\lambda}\left(\frac{x}{\lambda}\right)^{k-1}e^{-(x/\lambda)^k}, \quad x \ge 0 $$

Cumulative distribution function (c.d.f):

$$ F(x) = 1 - e^{-(x/\lambda)^k}, \quad x \ge 0 $$

Moment generating function (m.g.f):

$$ E(e^{tX}) = \sum_{n=0}^\infty \frac{t^n\lambda^n}{n!} \Gamma\left(1+\frac{n}{k}\right) $$

Skewness:

$$ \gamma_1 = \frac{\mu^3 - 3\mu\sigma^2 - \mu^3}{\sigma^3} $$

where \(\mu = E(X)\), \(\sigma^2 = \text{Var}(X)\), and the third raw moment is

$$ \mu^3 = \lambda^3 \Gamma\left(1 + \frac{3}{k}\right) $$

Excess Kurtosis:

$$ \gamma_2 = \frac{\mu^4 - 4\gamma_1\mu\sigma^3 - 6\mu^2\sigma^2 - \mu^4}{\sigma^4} - 3 $$

where the fourth raw moment is

$$ \mu^4 = \lambda^4 \Gamma\left(1 + \frac{4}{k}\right) $$

See Also

stats::Weibull

Examples

dist <- dist_weibull(shape = c(0.5, 1, 1.5, 5), scale = rep(1, 4))

dist
mean(dist)
variance(dist)
skewness(dist)
kurtosis(dist)

generate(dist, 10)

density(dist, 2)
density(dist, 2, log = TRUE)

cdf(dist, 4)

quantile(dist, 0.7)