LambertW (version 0.6.4)

LambertW-utils: Utilities for Lambert W\( \times\) F Random Variables

Description

Density, distribution, quantile function and random number generation for a Lambert W \(\times\) \(F_X(x \mid \boldsymbol \beta)\) random variable with parameter \(\theta = (\alpha, \boldsymbol \beta, \gamma, \delta)\).

Following the usual R dqpr family of functions (e.g., rnorm, dnorm, ...) the Lambert W\( \times\) F utility functions work as expected: dLambertW evaluates the pdf at y, pLambertW evaluates the cdf at y, qLambertW is the quantile function, and rLambertW generates random samples from a Lambert W \(\times\) \(F_X(x \mid \boldsymbol \beta)\) distribution.

mLambertW computes the first 4 central/standardized moments of a Lambert W \(\times\) F. Works only for Gaussian distribution.

qqLambertW computes and plots the sample quantiles of the data y versus the theoretical Lambert W \(\times\) \(F\) theoretical quantiles given \(\theta\).

Usage

dLambertW(y, distname = NULL, theta = NULL, beta = NULL, gamma = 0,
  delta = 0, alpha = 1, input.u = NULL, tau = NULL,
  use.mean.variance = TRUE, log = FALSE)
mLambertW(theta = NULL, distname = c("normal"), beta, gamma = 0,
  delta = 0, alpha = 1)
pLambertW(q, distname, theta = NULL, beta = NULL, gamma = 0, delta = 0,
  alpha = 1, input.u = NULL, tau = NULL, log = FALSE,
  lower.tail = FALSE, use.mean.variance = TRUE)
qLambertW(p, distname = NULL, theta = NULL, beta = NULL, gamma = 0,
  delta = 0, alpha = 1, input.u = NULL, tau = NULL,
  is.non.negative = FALSE, use.mean.variance = TRUE)
qqLambertW(y, distname, theta = NULL, beta = NULL, gamma = 0, delta = 0,
  alpha = 1, plot.it = TRUE, use.mean.variance = TRUE, ...)
rLambertW(n, distname, theta = NULL, beta = NULL, gamma = 0, delta = 0,
  alpha = 1, return.x = FALSE, input.u = NULL, tau = NULL,
  use.mean.variance = TRUE)

Arguments

y, q

vector of quantiles.

distname

character; name of input distribution; see get_distnames.

theta

list; a (possibly incomplete) list of parameters alpha, beta, gamma, delta. complete_theta fills in default values for missing entries.

beta

numeric vector (deprecated); parameter \(\boldsymbol \beta\) of the input distribution. See check_beta on how to specify beta for each distribution.

gamma

scalar (deprecated); skewness parameter; default: 0.

delta

scalar or vector (length 2) (deprecated); heavy-tail parameter(s); default: 0.

alpha

scalar or vector (length 2) (deprecated); heavy tail exponent(s); default: 1.

input.u

users can supply their own version of U (either a vector of simulated values or a function defining the pdf/cdf/quanitle function of U); default: NULL. If not NULL, tau must be specified as well.

tau

optional; if input.u = TRUE, then tau must be specified. Note that \(\boldsymbol \beta\) is still taken from theta, but "mu_x", "sigma_x", and the other parameters (\(\alpha, \gamma, \delta\)) are all taken from tau. This is usually only used by the create_LambertW_output function; users usually don't need to supply this argument directly.

use.mean.variance

logical; if TRUE it uses mean and variance implied by \(\boldsymbol \beta\) to do the transformation (Goerg 2011). If FALSE, it uses the alternative definition from Goerg (2016) with location and scale parameter.

log

logical; if TRUE, probabilities p are given as log(p).

lower.tail

logical; if TRUE (default), probabilities are \(P(X \leq x)\) otherwise, \(P(X > x)\).

p

vector of probability levels

is.non.negative

logical; by default it is set to TRUE if the distribution is not a location but a scale family.

plot.it

logical; should the result be plotted? Default: TRUE.

n

number of observations

return.x

logical; if TRUE not only the simulated Lambert W\( \times\) F sample y, but also the corresponding simulated input x will be returned. Default FALSE. Note: if TRUE then rLambertW does not return a vector of length n, but a list of two vectors (each of length n).

…

further arguments passed to or from other methods.

Value

mLambertW returns a list with the 4 theoretical (central/standardized) moments of \(Y\) implied by \(\boldsymbol \theta\) and distname (currrently, this only works for distname = "normal"):

mean

mean,

sd

standard deviation,

skewness

skewness,

kurtosis

kurtosis (not excess kurtosis, i.e., 3 for a Gaussian).

rLambertW returns a vector of length n. If return.input = TRUE, then it returns a list of two vectors (each of length n):

x

simulated input,

y

Lambert W random sample (transformed from x - see References and get_output).

qqLambertW returns a list of 2 vectors (analogous to qqnorm):

x

theoretical quantiles (sorted),

y

empirical quantiles (sorted).

Details

All functions here have an optional input.u argument where users can supply their own version corresponding to zero-mean, unit variance input \(U\). This function usually depends on the input parameter \(\boldsymbol \beta\); e.g., users can pass their own density function dmydist <- function(u, beta) {...} as dLambertW(..., input.u = dmydist). dLambertW will then use this function to evaluate the pdf of the Lambert W x 'mydist' distribution.

Important: Make sure that all input.u in dLambertW, pLambertW, ... are supplied correctly and return correct values -- there are no unit-tests or sanity checks for user-defined functions.

See the references for the analytic expressions of the pdf and cdf. For "h" or "hh" types and for scale-families of type = "s" quantiles can be computed analytically. For location (-scale) families of type = "s" quantiles need to be computed numerically.

Examples

# NOT RUN {
###############################
######### mLambertW ###########
mLambertW(theta = list(beta = c(0, 1), gamma = 0.1))
mLambertW(theta = list(beta = c(1, 1), gamma = 0.1)) # mean shifted by 1
mLambertW(theta = list(beta = c(0, 1), gamma = 0)) # N(0, 1)

###############################
######### rLambertW ###########
set.seed(1)
# same as rnorm(1000)
x <- rLambertW(n=100, theta = list(beta=c(0, 1)), distname = "normal") 
skewness(x) # very small skewness
medcouple_estimator(x) # also close to zero

y <- rLambertW(n=100, theta = list(beta = c(1, 3), gamma = 0.1), 
               distname = "normal")
skewness(y) # high positive skewness (in theory equal to 3.70)
medcouple_estimator(y) # also the robust measure gives a high value

op <- par(no.readonly=TRUE)
par(mfrow = c(2, 2), mar = c(2, 4, 3, 1))
plot(x)
hist(x, prob=TRUE, 15)
lines(density(x))

plot(y)
hist(y, prob=TRUE, 15)
lines(density(y))
par(op)
###############################
######### dLambertW ###########
beta.s <- c(0, 1)
gamma.s <- 0.1

# x11(width=10, height=5)
par(mfrow = c(1, 2), mar = c(3, 3, 3, 1))
curve(dLambertW(x, theta = list(beta = beta.s, gamma = gamma.s), 
                distname = "normal"),
     -3.5, 5, ylab = "",  main="Density function")
plot(dnorm, -3.5, 5, add = TRUE, lty = 2)
legend("topright" , c("Lambert W x Gaussian" , "Gaussian"), lty = 1:2)
abline(h=0)

###############################
######### pLambertW ###########

curve(pLambertW(x, theta = list(beta = beta.s, gamma = gamma.s),
                distname = "normal"),
      -3.5, 3.5, ylab = "", main = "Distribution function")
plot(pnorm, -3.5,3.5, add = TRUE, lty = 2)
legend("topleft" , c("Lambert W x Gaussian" , "Gaussian"), lty = 1:2)
par(op)

######## Animation 
# }
# NOT RUN {
gamma.v <- seq(-0.15, 0.15, length = 31) # typical, empirical range of gamma
b <- get_support(gamma_01(min(gamma.v)))[2]*1.1
a <- get_support(gamma_01(max(gamma.v)))[1]*1.1

for (ii in seq_along(gamma.v)) {
  curve(dLambertW(x, beta = gamma_01(gamma.v[ii])[c("mu_x", "sigma_x")], 
                  gamma = gamma.v[ii], distname="normal"),
        a, b, ylab="", lty = 2, col = 2, lwd = 2, main = "pdf", 
        ylim = c(0, 0.45))
  plot(dnorm, a, b, add = TRUE, lty = 1, lwd = 2)
  legend("topright" , c("Lambert W x Gaussian" , "Gaussian"), 
         lty = 2:1, lwd = 2, col = 2:1)
  abline(h=0)
  legend("topleft", cex = 1.3, 
         c(as.expression(bquote(gamma == .(round(gamma.v[ii],3))))))
Sys.sleep(0.04)
}
# }
# NOT RUN {
###############################
######### qLambertW ###########

p.v <- c(0.01, 0.05, 0.5, 0.9, 0.95,0.99)
qnorm(p.v)
# same as above except for rounding errors
qLambertW(p.v, theta = list(beta = c(0, 1), gamma = 0), distname = "normal") 
# positively skewed data -> quantiles are higher
qLambertW(p.v, theta = list(beta = c(0, 1), gamma = 0.1),
          distname = "normal")

###############################
######### qqLambertW ##########
# }
# NOT RUN {
y <- rLambertW(n=500, distname="normal", 
               theta = list(beta = c(0,1), gamma = 0.1))

layout(matrix(1:2, ncol = 2))
qqnorm(y)
qqline(y)
qqLambertW(y, theta = list(beta = c(0, 1), gamma = 0.1), 
           distname = "normal") 
# }