# SICHEL

0th

Percentile

##### The Sichel distribution for fitting a GAMLSS model

The SICHEL() function defines the Sichel distribution, a three parameter discrete distribution, for a gamlss.family object to be used in GAMLSS fitting using the function gamlss(). The functions dSICHEL, pSICHEL, qSICHEL and rSICHEL define the density, distribution function, quantile function and random generation for the Sichel SICHEL(), distribution. The function VSICHEL gives the variance of a fitted Sichel model.

The functions ZASICHEL() and ZISICHEL() are the zero adjusted (hurdle) and zero inflated versions of the Sichel distribution, respectively. That is four parameter distributions.

The functions dZASICHEL, dZISICHEL, pZASICHEL,pZISICHEL, qZASICHEL qZISICHEL rZASICHEL and rZISICHEL define the probability, cumulative, quantile and random generation functions for the zero adjusted and zero inflated Sichel distributions, ZASICHEL(), ZISICHEL(), respectively.

Keywords
distribution, regression
##### Usage
SICHEL(mu.link = "log", sigma.link = "log", nu.link = "identity")
dSICHEL(x, mu=1, sigma=1, nu=-0.5, log=FALSE)
pSICHEL(q, mu=1, sigma=1, nu=-0.5, lower.tail = TRUE,
log.p = FALSE)
qSICHEL(p, mu=1, sigma=1, nu=-0.5,  lower.tail = TRUE,
log.p = FALSE, max.value = 10000)
rSICHEL(n, mu=1, sigma=1, nu=-0.5, max.value = 10000)
VSICHEL(obj)
dZASICHEL(x, mu = 1, sigma = 1, nu = -0.5, tau = 0.1, log = FALSE)
pZASICHEL(q, mu = 1, sigma = 1, nu = -0.5, tau = 0.1,
lower.tail = TRUE, log.p = FALSE)
qZASICHEL(p, mu = 1, sigma = 1, nu = -0.5, tau = 0.1,
lower.tail = TRUE, log.p = FALSE, max.value = 10000)
rZASICHEL(n, mu = 1, sigma = 1, nu = -0.5, tau = 0.1,
dZISICHEL(x, mu = 1, sigma = 1, nu = -0.5, tau = 0.1, log = FALSE)
pZISICHEL(q, mu = 1, sigma = 1, nu = -0.5, tau = 0.1,
lower.tail = TRUE, log.p = FALSE)
qZISICHEL(p, mu = 1, sigma = 1, nu = -0.5, tau = 0.1,
lower.tail = TRUE, log.p = FALSE, max.value = 10000)
rZISICHEL(n, mu = 1, sigma = 1, nu = -0.5, tau = 0.1,
max.value = 10000)
##### Arguments

Defines the mu.link, with "log" link as the default for the mu parameter

Defines the sigma.link, with "log" link as the default for the sigma parameter

Defines the nu.link, with "identity" link as the default for the nu parameter

Defines the tau.link, with "logit" link as the default for the tau parameter

x

vector of (non-negative integer) quantiles

mu

vector of positive mu

sigma

vector of positive dispersion parameter sigma

nu

vector of nu

tau

vector of probabilities tau

p

vector of probabilities

q

vector of quantiles

n

number of random values to return

log, log.p

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

lower.tail

logical; if TRUE (default), probabilities are P[X <= x], otherwise, P[X > x]

max.value

a constant, set to the default value of 10000 for how far the algorithm should look for q

obj

a fitted Sichel gamlss model

y

the y variable, the tofySICHEL() should not be used on its own.

##### Details

The probability function of the Sichel distribution is given by $$f(y|\mu,\sigma,\nu)= \frac{\mu^y K_{y+\nu}(\alpha)}{y!(\alpha \sigma)^{y+\nu} K_\nu(\frac{1}{\sigma})}$$

for $y=0,1,2,...,\infty$, $\mu>0$ , $\sigma>0$ and $-\infty <\nu<\infty$ where

$$\alpha^2=\frac{1}{\sigma^2}+\frac{2\mu}{\sigma}$$

$$c=K_{\nu+1}(1/\sigma) / K_{\nu}(1/\sigma)$$ and $K_{\lambda}(t)$ is the modified Bessel function of the third kind. Note that the above parametrization is different from Stein, Zucchini and Juritz (1988) who use the above probability function but treat $\mu$, $\alpha$ and $\nu$ as the parameters.

##### Value

Returns a gamlss.family object which can be used to fit a Sichel distribution in the gamlss() function.

##### Note

The mean of the above Sichel distribution is $\mu$ and the variance is $\mu^2 \left[\frac{2\sigma (\nu+1)}{c} + \frac{1}{c^2}-1\right]$

##### References

Rigby, R. A. and Stasinopoulos D. M. (2005). Generalized additive models for location, scale and shape,(with discussion), Appl. Statist., 54, part 3, pp 507-554.

Rigby, R. A., Stasinopoulos D. M. and Akantziliotou, C. (2006) Modelling the parameters of a family of mixed Poisson distributions including the Sichel and Delaptorte. Submitted for publication.

Stasinopoulos D. M., Rigby R.A. and Akantziliotou C. (2003) Instructions on how to use the GAMLSS package in R. Accompanying documentation in the current GAMLSS help files, (see also http://www.gamlss.org/).

Stasinopoulos D. M. Rigby R.A. (2007) Generalized additive models for location scale and shape (GAMLSS) in R. Journal of Statistical Software, Vol. 23, Issue 7, Dec 2007, http://www.jstatsoft.org/v23/i07.

Stasinopoulos D. M., Rigby R.A., Heller G., Voudouris V., and De Bastiani F., (2017) Flexible Regression and Smoothing: Using GAMLSS in R, Chapman and Hall/CRC.

Stein, G. Z., Zucchini, W. and Juritz, J. M. (1987). Parameter Estimation of the Sichel Distribution and its Multivariate Extension. Journal of American Statistical Association, 82, 938-944.

gamlss.family, PIG , SI

• SICHEL
• dSICHEL
• pSICHEL
• qSICHEL
• rSICHEL
• VSICHEL
• tofySICHEL
• ZASICHEL
• dZASICHEL
• pZASICHEL
• qZASICHEL
• rZASICHEL
• ZISICHEL
• dZISICHEL
• pZISICHEL
• qZISICHEL
• rZISICHEL
##### Examples
# NOT RUN {
#plot the pdf using plot
plot(function(y) dSICHEL(y, mu=10, sigma=1, nu=1), from=0, to=100, n=100+1, type="h") # pdf
# plot the cdf
plot(seq(from=0,to=100),pSICHEL(seq(from=0,to=100), mu=10, sigma=1, nu=1), type="h")   # cdf
# generate random sample
tN <- table(Ni <- rSICHEL(100, mu=5, sigma=1, nu=1))
r <- barplot(tN, col='lightblue')
# fit a model to the data
# library(gamlss)
# gamlss(Ni~1,family=SICHEL, control=gamlss.control(n.cyc=50))
# }

Documentation reproduced from package gamlss.dist, version 5.1-1, License: GPL-2 | GPL-3

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