evmix (version 1.0)

gammagpd: Gamma Bulk and GPD Tail Extreme Value Mixture Model

Description

Density, cumulative distribution function, quantile function and random number generation for the extreme value mixture model with gamma for bulk distribution upto the threshold and conditional GPD above threshold. The parameters are the gamma shape gshape and scale gscale, threshold u GPD scale sigmau and shape xi and tail fraction phiu.

Usage

dgammagpd(x, gshape = 1, gscale = 1,
    u = qgamma(0.9, gshape, 1/gscale),
    sigmau = sqrt(gshape) * gscale, xi = 0, phiu = TRUE,
    log = FALSE)

  pgammagpd(q, gshape = 1, gscale = 1,
    u = qgamma(0.9, gshape, 1/gscale),
    sigmau = sqrt(gshape) * gscale, xi = 0, phiu = TRUE,
    lower.tail = TRUE)

  qgammagpd(p, gshape = 1, gscale = 1,
    u = qgamma(0.9, gshape, 1/gscale),
    sigmau = sqrt(gshape) * gscale, xi = 0, phiu = TRUE,
    lower.tail = TRUE)

  rgammagpd(n = 1, gshape = 1, gscale = 1,
    u = qgamma(0.9, gshape, 1/gscale),
    sigmau = sqrt(gshape) * gscale, xi = 0, phiu = TRUE)

Arguments

gshape

gamma shape (non-negative)

gscale

gamma scale (non-negative)

u

threshold (non-negative)

phiu

probability of being above threshold [0,1] or TRUE

x

quantile

sigmau

scale parameter (non-negative)

xi

shape parameter

log

logical, if TRUE then log density

q

quantile

lower.tail

logical, if FALSE then upper tail probabilities

p

cumulative probability

n

sample size (non-negative integer)

Value

• dgammagpd gives the density, pgammagpd gives the cumulative distribution function, qgammagpd gives the quantile function and rgammagpd gives a random sample.

Details

Extreme value mixture model combining gamma distribution for the bulk below the threshold and GPD for upper tail. The user can pre-specify phiu permitting a parameterised value for the tail fraction $\phi_u$. Alternatively, when phiu=TRUE the tail fraction is estimated as the tail fraction from the gamma bulk model. The cumulative distribution function with tail fraction $\phi_u$ defined by the upper tail fraction of the gamma bulk model (phiu=TRUE), upto the threshold $0 < x \le u$, given by: $$F(x) = H(x)$$ and above the threshold $x > u$: $$F(x) = H(u) + [1 - H(u)] G(x)$$ where $H(x)$ and $G(X)$ are the gamma and conditional GPD cumulative distribution functions (i.e. pgamma(x, gshape, scale = gscale) and pgpd(x, u, sigmau, xi)). The cumulative distribution function for pre-specified $\phi_u$, upto the threshold $0 < x \le u$, is given by: $$F(x) = (1 - \phi_u) H(x)/H(u)$$ and above the threshold $x > u$: $$F(x) = \phi_u + [1 - \phi_u] G(x)$$ Notice that these definitions are equivalent when $\phi_u = 1 - H(u)$. The gamma is defined on the non-negative reals, so the threshold must be non-negative. See gpd for details of GPD upper tail component and dgamma for details of gamma bulk component.

References

http://en.wikipedia.org/wiki/Gamma_distribution http://en.wikipedia.org/wiki/Generalized_Pareto_distribution Scarrott, C.J. and MacDonald, A. (2012). A review of extreme value threshold estimation and uncertainty quantification. REVSTAT - Statistical Journal 10(1), 33-59. Available from http://www.ine.pt/revstat/pdf/rs120102.pdf Behrens, C.N., Lopes, H.F. and Gamerman, D. (2004). Bayesian analysis of extreme events with threshold estimation. Statistical Modelling. 4(3), 227-244.

See Also

gpd and dgamma Other gammagpd: fgammagpd, lgammagpd, nlgammagpd

Examples

par(mfrow=c(2,2))
x = rgammagpd(1000, gshape = 2)
xx = seq(-1, 10, 0.01)
hist(x, breaks = 100, freq = FALSE, xlim = c(-1, 10))
lines(xx, dgammagpd(xx, gshape = 2))

# three tail behaviours
plot(xx, pgammagpd(xx, gshape = 2), type = "l")
lines(xx, pgammagpd(xx, gshape = 2, xi = 0.3), col = "red")
lines(xx, pgammagpd(xx, gshape = 2, xi = -0.3), col = "blue")
legend("topleft", paste("xi =",c(0, 0.3, -0.3)),
  col=c("black", "red", "blue"), lty = 1)

x = rgammagpd(1000, gshape = 2, u = 3, phiu = 0.2)
hist(x, breaks = 100, freq = FALSE, xlim = c(-1, 10))
lines(xx, dgammagpd(xx, gshape = 2, u = 3, phiu = 0.2))

plot(xx, dgammagpd(xx, gshape = 2, u = 3, xi=0, phiu = 0.2), type = "l")
lines(xx, dgammagpd(xx, gshape = 2, u = 3, xi=-0.2, phiu = 0.2), col = "red")
lines(xx, dgammagpd(xx, gshape = 2, u = 3, xi=0.2, phiu = 0.2), col = "blue")
legend("topright", c("xi = 0", "xi = 0.2", "xi = -0.2"),
  col=c("black", "red", "blue"), lty = 1)