GHcop: The Gumbel-Hougaard Extreme Value Copula
In wasquith/copBasic: General Bivariate Copula Theory and Many Utility Functions
GHcop R Documentation
The Gumbel–Hougaard Extreme Value Copula
Description
SYMMETRIC GUMBEL-HOUGAARD—The Gumbel–Hougaard copula (Nelsen, 2006, pp. 118 and 164) is
\mathbf{C}_{\Theta}(u,v) = \mathbf{GH}(u,v) = \mathrm{exp}\{-[(-\log u)^\Theta+(-\log v)^\Theta]^{1/\Theta}\}\mbox{,}
where \Theta \in [1 , \infty). The copula here is a bivariate extreme value copula (BEV). The parameter \Theta is readily estimated using a Kendall Tau (say a sample version \hat\tau) where the \tau of the copula (\tau_\mathbf{C}) is defined as
\tau_\mathbf{C} = \frac{\Theta - 1}{\Theta} \rightarrow \Theta = \frac{1}{1-\tau}\mbox{.}
The copula is readily extended into d dimensions by
\mathbf{C}_{\Theta}(u,v) = \mathrm{exp}\{-[(-\log u_1)^\Theta+\cdots+(-\log u_d)^\Theta]^{1/\Theta}\}\mbox{.}
However, such an implementation is not available in the copBasic package.
Every Gumbel–Hougaard copula is a multivariate extreme value (MEV) copula, and hence useful in analysis of extreme value distributions. The Gumbel–Hougaard copula is the only Archimedean MEV (Salvadori et al., 2007, p. 192). The Gumbel–Hougaard copula has respective lower- and upper-tail dependency parameters of \lambda^L = 0 and \lambda^U = 2 - 2^{1/\Theta}, respectively. Nelsen (2006, p. 96) shows that \mathbf{C}^r_\theta(u^{1/r}, v^{1/r}) = \mathbf{C}_\theta(u,v) so that every Gumbel–Hougaard copula has a property known as max-stable. A dependence measure uniquely defined for BEV copulas is shown under rhobevCOP.
A comparison through simulation between Gumbel–Hougaard implementations by the R packages acopula, copBasic, copula, and Gumbel is shown in the Examples section. At least three divergent techniques for random variate generation are used amongst those packages. The simulations also use copBasic-style random variate generation (conditional simulation) using an analytical-numerical hybrid solution to conditional inverse described in the Note section.
TWO-PARAMETER GUMBEL–HOUGAARD—A permutation symmetric (isCOP.permsym) but almost certainly radial asymmetric (isCOP.radsym) version of the copula is readily constructed (Brahimi et al., 2015) into a two-parameter version:
\mathbf{C}(u,v; \beta_1, \beta_2) =
\biggl[
\biggl(\bigl(u^{-\beta_2} -1\bigr)^{\beta_1} +
\bigl(v^{-\beta_2} -1\bigr)^{\beta_1}
\biggr)^{1/\beta_1} + 1
\biggr]^{-1/\beta_2}\mbox{,}
where \beta_1 \ge 1 and \beta_2 > 0. Both parameters controls the general level of association, whereas parameter \beta_2 can be thought of as controlling left-tail dependency (taildepCOP, \lambda^{[U\mid L]}_{(\beta_1, \beta_2)}; e.g.
\lambda^U_{(1.5; \beta_2)} = 0.413 for all \beta_2 but
\lambda^L_{(1.5; 0.2)} = 0.811 and
\lambda^L_{(1.5; 2.2)} = 0.099. Brahimi et al. (2015) report a Spearman Rho (rhoCOP) for a \mathbf{GH}_{(1.5, 0.2)}(u,v) is 0.5, which is readily confirmed in copBasic by the function call rhoCOP(cop=GHcop, para=c(1.5,0.2)). The two-parameter \mathbf{GH} is triggered if the length of the para argument is exactly 2.
ASYMMETRIC GUMBEL–HOUGAARD—An asymmetric version of the copula is readily constructed (Joe, 2014, p. 185–186) into a three-parameter version with Marshall–Olkin copulas on the boundaries:
\mathbf{C}(u,v; \Theta, \pi_2, \pi_3) = \mathrm{exp}[-\mathcal{A}(-\log u, -\log v; \Theta, \pi_2, \pi_3)]\mbox{,}
where \Theta \ge 1 as before, 0 \le \pi_2, \pi_3 \le 1, and
\mathcal{A}(x, y; \Theta, \pi_2, \pi_3) = [(\pi_2 x)^\Theta + (\pi_3 y)^\Theta]^{1/\Theta} + (1-\pi_2)x + (1-\pi_3)y\mbox{.}
The asymmetric \mathbf{GH} is triggered if the length of the para argument is exactly 3. The GHcop function provides no mechanism for estimation of the parameters for the asymmetric version. Reviewing simulations, the bounds on the \pi parameters in Joe (2014, p. 185) “[0 \le \pi_2 < \pi_3 \le 1]” might be incorrect—by Joe back referencing to Joe (2014, eq. 4.35, p. 183) the \pi-limits as stated for copBasic are shown. An algorithm for parameter estimation for the asymmetric \mathbf{GH} using two different measures of bivariate skewness as well as an arbitrary measure of association is shown in section Details in joeskewCOP.
Usage
GHcop(u, v, para=NULL, tau=NULL, tau.big=0.985, cor=NULL, ...)
Arguments
u
Nonexceedance probability u in the X direction;
v
Nonexceedance probability v in the Y direction;
para
A vector (single element or triplet) of parameters—the \Theta parameter of the copula;
tau
Kendall Tau \tau from which to estimate the parameter \Theta;
tau.big
The largest value for \tau_\mathbf{C} prior to switching to the \mathbf{M} copula applicable to the the symmetric version of this copula;
cor
A copBasic syntax for “the correlation coefficient” suitable for the copula—a synonym for tau; and
...
Additional arguments to pass.
Details
Numerical experiments seem to indicate for \tau_\mathbf{C} > 0.985 that failures in the numerical partial derivatives in derCOP and derCOP2 result—a \tau_\mathbf{C} this large is indeed large. As \Theta \rightarrow \infty the Gumbel–Hougaard copula becomes the Fréchet–Hoeffding upper-bound copula \mathbf{M} (see M). A \tau_\mathbf{C} \approx 0.985 yields \Theta \approx 66 + 2/3, then for \Theta > 1/(1-\tau_\mathbf{C}) flips over to the \mathbf{M} copula with a warning issued.
Value
Value(s) for the copula are returned using the \Theta as set by argument para. Alternative returned values are possible: (1) If para=NULL and tau is set, then \tau_\mathbf{C} \rightarrow \Theta and an R list is returned. (2) If para=NULL and tau=NULL, then an attempt to estimate \Theta from the u and v is made by \mathrm{cor}(u,v)_\tau \rightarrow \tau_\mathbf{C} \rightarrow \Theta by either trigger using cor(u,v, method="kendall") in R, and an R list is returned. The possibly returned list has the following elements:
para
The computed \Theta from the given bivariate data in para; and
tau
The sample estimate of \tau.
Note
SYMMETRIC GUMBEL–HOUGAARD—A function for the derivative of the copula (Joe, 2014, p. 172) given u is
"GHcop.derCOP" <- function(u, v, para=NULL, ...) {
x <- -log(u); y <- -log(v)
A <- exp(-(x^para + y^para)^(1/para)) * (1 + (y/x)^para)^(1/para - 1)
return(A/u)
}
that can be tested by the following
Theta <- 1/(1-.15) # a Kendall Tau of 0.15
GHcop.derCOP( 0.5, 0.75, para=Theta) # 0.7787597
derCOP(cop=GHcop, 0.5, 0.75, para=Theta) # 0.7787597
# The next two nearly return same value but conversion to GRVs
# (Gumbel Reduced Variates) to magnify the numerical differences.
# The GHcop.derCOP is expected to be the more accurate of the two.
lmomco::prob2grv(GHcop.derCOP( 0.5, 0.9999999, para=Theta)) # 18.83349
lmomco::prob2grv(derCOP(cop=GHcop, 0.5, 0.9999999, para=Theta)) # 18.71497
lmomco::prob2grv(derCOP(cop=GHcop, 0.5, 0.9999999, para=Theta,
delu=.Machine$double.eps^.25)) # 18.83341
where the last numerical approximation shows that tighter tolerance is needed. A function for the inverse of the derivative (Joe, 2014, p. 172) given u by an analytical-numerical hybrid is
"GHcop.derCOPinv" <- function(u,t, para=NULL, verbose=FALSE,
tol=.Machine$double.eps, ...) {
if(length(u) > 1) warning("only the first value of u will be used")
if(length(t) > 1) warning("only the first value of t will be used")
if(is.null(para)) { warning("para can not be NULL"); return(NA) }
u <- u[1]; t <- t[1]; rt <- NULL
x <- -log(u); A <- (x + (para - 1)*log(x) - log(t))
hz <- function(z) { z + (para - 1)*log(z) - A }
zmax <- x; i <- 0; hofz.lo <- hz(zmax)
if(sign(hofz.lo) != -1) warning("sign for h(z) is not negative!")
while(1) {
i <- i + 1
if(i > 100) {
warning("maximum iterations looking for zmax reached"); break
}
# increment zmax by 1/2 log cycle, sign(hofz.lo) should be negative!
if(sign(hz(zmax <- zmax + 1/2)) != sign(hofz.lo)) break
}
try(rt <- uniroot(hz, c(x, zmax), tol=tol, ...), silent=FALSE)
if(verbose) print(rt)
if(is.null(rt)) {
warning("NULL on the inversion of the GH copula derivative")
return(NA)
}
zo <- rt$root
y <- (zo^para - x^para)^(1/para)
names(y) <- NULL
return(exp(-y))
}
that can be tested by the following, which also shows how to increase the tolerance on the numerical implementation
u <- 0.999; p <- 0.999
GHcop.derCOPinv( u, p, para=1.56) # 0.999977
derCOPinv(cop=GHcop, u, p, para=1.56) # 1 (unity), needs tighter tolerance
derCOPinv(cop=GHcop, u, p, para=1.56, tol=.Machine$double.eps/10) # 0.999977
ASYMMETRIC GUMBEL–HOUGAARD—Set \tau_\mathbf{C} = 0.35 then for a symmetric and then reflection on the 1:1 line of the asymmetric Gumbel–Hougaard copula and compute the primary parameter \Theta, and lastly, compute three bivariate \nu_\mathbf{C} skewnesses (nuskewCOP):
Theta1 <- uniroot(function(t) {
0.35 - tauCOP(cop=GHcop, para=c(t)) }, c(1,10))$root
Theta2 <- uniroot(function(t) { # asymmetric
0.35 - tauCOP(cop=GHcop, para=c(t, 0.6, 0.9)) }, c(1,30))$root
Theta3 <- uniroot(function(t) { # asymmetric reflection on 1:1
0.35 - tauCOP(cop=GHcop, para=c(t, 0.9, 0.6)) }, c(1,30))$root
# Theta1 = 1.538462 and Theta2 = Theta3 = 2.132856
# Three "skews" based on a combination of U, V, and C(u,v) [nuskew()]
nuskewCOP(cop=GHcop, 1.538462) # zero bivariate skewness
nuskewCOP(cop=GHcop, c(2.132856, 0.6, 0.9)) # 0.008245653
nuskewCOP(cop=GHcop, c(2.132856, 0.9, 0.6)) # -0.008245653
So, we see, holding \tau_\mathbf{C} constant, that the \mathbf{GH} has a \nu_{\mathbf{GH}(1.538)} = 0 but the asymmetric case \nu_{\mathbf{GH}(2.133, 0.6, 0.9)} = 0.0082 and \nu_{\mathbf{GH}(2.133, 0.9, 0.6)} = -0.0082 where the change in sign represents reflection about the 1:1 line. Finally, compute L-coskew by large simulation and the adjective “bow” representing the direction of bowing or curvature of the principle copula density.
# Because the Tau's are all similar, there is nothing to learn from the
# L-correlation, let us inspect the L-coskew instead:
coT3.1<-lmomco::lcomoms2(simCOP(n=8000, cop=GHcop, para=c(Theta1 )))$T3
coT3.2<-lmomco::lcomoms2(simCOP(n=8000, cop=GHcop, para=c(Theta2,.6,.9)))$T3
coT3.3<-lmomco::lcomoms2(simCOP(n=8000, cop=GHcop, para=c(Theta3,.9,.6)))$T3
# The simulations for Theta1 have no curvature about the diagonal.
# The simulations for Theta2 have curvature towards the upper left.
# The simulations for Theta3 have curvature towards the lower right.
message("# L-coskews: ",round(coT3.1[1,2],digits=4),"(symmetric) ",
round(coT3.2[1,2],digits=4),"(asym.--bow UL) ",
round(coT3.3[1,2],digits=4),"(asym.--bow UL)")
message("# L-coskews: ",round(coT3.1[2,1],digits=4),"(symmetric) ",
round(coT3.2[2,1],digits=4),"(asym.--bow LR) ",
round(coT3.3[2,1],digits=4),"(asym.--bow LR)")
# L-coskews: 0.0533(symmetric) 0.1055(asym.--bow UL) 0.0021(asym.--bow UL)
# L-coskews: 0.0679(symmetric) 0.0112(asym.--bow LR) 0.1154(asym.--bow LR)
Thus, the L-comoments (Asquith, 2011) using their sample values measure something fundamental about the bivariate association between the three copulas choosen. The L-coskews for the symmetrical case are about equal and are
\tau^{\mathbf{GH}(\Theta_1)}_{3[12]} \approx \tau^{\mathbf{GH}(\Theta_1)}_{3[21]} \rightarrow (0.0533 + 0.0679)/2 = 0.0606\mbox{,}
whereas the L-coskew for the curvature to the upper left are
\tau^{\mathbf{GH}(\Theta_2, 0.6, 0.9)}_{3[12]} = 0.1055\mbox{\ and\ } \tau^{\mathbf{GH}(\Theta_2, 0.6, 0.9)}_{3[12]} = 0.0112\mbox{,}
whereas the L-coskew for the curvature to the lower right are
\tau^{\mathbf{GH}(\Theta_3, 0.9, 0.6)}_{3[12]} = 0.0021\mbox{\ and\ } \tau^{\mathbf{GH}(\Theta_3, 0.6, 0.9)}_{3[12]} = 0.1154\mbox{.}
Thus, the \pi_2 and \pi_3 parameters as choosen add about ((0.1154+0.1055)/2 - 0.0606 \rightarrow 0.1105 - 0.0606 = 0.05) L-coskew units to the bivariate distribution.
Author(s)
W.H. Asquith
References
Asquith, W.H., 2011, Distributional analysis with L-moment statistics using the R environment for statistical computing: Createspace Independent Publishing Platform, ISBN 978–146350841–8.
Brahimi, B., Chebana, F., and Necir, A., 2015, Copula representation of bivariate L-moments—A new estimation method for multiparameter two-dimensional copula models: Statistics, v. 49, no. 3, pp. 497–521.
Joe, H., 2014, Dependence modeling with copulas: Boca Raton, CRC Press, 462 p.
Nelsen, R.B., 2006, An introduction to copulas: New York, Springer, 269 p.
Salvadori, G., De Michele, C., Kottegoda, N.T., and Rosso, R., 2007, Extremes in Nature—An approach using copulas: Springer, 289 p.
Zhang, L., and Singh, V.P., 2007, Gumbel–Hougaard copula for trivariate rainfall frequency analysis: Journal Hydrologic Engineering, v. 12, Special issue—Copulas in Hydrology, pp. 409–419.
See Also
M, GLcop, HRcop, tEVcop, rhobevCOP
Examples
Theta <- 2.2 # Let us see if numerical and analytical tail deps are the same.
del.lamU <- abs( taildepCOP(cop=GHcop, para=Theta)$lambdaU - (2-2^(1/Theta)) )
as.logical(del.lamU < 1E-6) # TRUE
## Not run:
# The simulations match Joe (2014, p. 72) for Gumbel-Hougaard
n <- 600; nsim <- 1000; set.seed(946) # see for reproducibility
SM <- sapply(1:nsim, function(i) { rs <- semicorCOP(cop=GHcop, para=1.35, n=n)
c(rs$botleft.semicor, rs$topright.semicor) })
RhoM <- round(mean(SM[1,]), digits=3)
RhoP <- round(mean(SM[2,]), digits=3)
SE.RhoM <- round( sd(SM[1,]), digits=3)
SE.RhoP <- round( sd(SM[2,]), digits=3)
SE.RhoMP <- round( sd(SM[2,] - SM[1,]), digits=3)
# Semi-correlations (sRho) and standard errors (SEs)
message("# sRho[-]=", RhoM, " (SE[-]=", SE.RhoM, ") Joe(p.72)=0.132 (SE[-]=0.08)")
message("# sRho[+]=", RhoP, " (SE[+]=", SE.RhoP, ") Joe(p.72)=0.415 (SE[+]=0.07)")
message("# SE(sRho[-] - sRho[+])=", SE.RhoMP, " Joe(p.72) SE=0.10")
# sRho[-]=0.134 (SE[-]=0.076) Joe(p.72)=0.132 (SE[-]=0.08)
# sRho[+]=0.407 (SE[+]=0.074) Joe(p.72)=0.415 (SE[+]=0.07)
# SE(sRho[-] - sRho[+])=0.107 Joe(p.72) SE=0.10
# Joe (2014, p. 72) reports the values 0.132, 0.415, 0.08, 0.07, 0.10, respectively.
## End(Not run)
## Not run:
file <- "Lcomom_study_of_GHcopPLACKETTcop.txt"
x <- data.frame(tau=NA, trho=NA, srho=NA, PLtheta=NA, PLT2=NA, PLT3=NA, PLT4=NA,
GHtheta=NA, GHT2=NA, GHT3=NA, GHT4=NA )
write.table(x, file=file, row.names=FALSE, quote=FALSE)
n <- 250 # Make a large number for very long CPU run but seems stable
for(tau in seq(0,0.98, by=0.005)) {
thetag <- GHcop(u=NULL, v=NULL, tau=tau)$para
trho <- rhoCOP(cop=GHcop, para=thetag)
GH <- simCOP(n=n, cop=GHcop, para=thetag, points=FALSE, ploton=FALSE)
srho <- cor(GH$U, GH$V, method="spearman")
thetap <- PLACKETTpar(rho=trho)
PL <- simCOP(n=n, cop=PLACKETTcop, para=thetap, points=FALSE, ploton=FALSE)
GHl <- lmomco::lcomoms2(GH, nmom=4); PLl <- lmomco::lcomoms2(PL, nmom=4)
x <- data.frame(tau=tau, trho=trho, srho=srho,
GHtheta=thetag, PLtheta=thetap,
GHT2=mean(c(GHl$T2[1,2], GHl$T2[2,1])),
GHT3=mean(c(GHl$T3[1,2], GHl$T3[2,1])),
GHT4=mean(c(GHl$T4[1,2], GHl$T4[2,1])),
PLT2=mean(c(PLl$T2[1,2], PLl$T2[2,1])),
PLT3=mean(c(PLl$T3[1,2], PLl$T3[2,1])),
PLT4=mean(c(PLl$T4[1,2], PLl$T4[2,1])) )
write.table(x, file=file, row.names=FALSE, col.names=FALSE, append=TRUE)
}
# After a processing run with very large "n", then meaningful results exist.
D <- read.table(file, header=TRUE); D <- D[complete.cases(D),]
plot(D$tau, D$GHT3, ylim=c(-0.08,0.08), type="n",
xlab="KENDALL TAU", ylab="L-COSKEW OR NEGATED L-COKURTOSIS")
points(D$tau, D$GHT3, col=2); points(D$tau, D$PLT3, col=1)
points(D$tau, -D$GHT4, col=4, pch=2); points(D$tau, -D$PLT4, col=1, pch=2)
LM3 <- lm(D$GHT3~I(D$tau^1)+I(D$tau^2)+I(D$tau^4)-1)
LM4 <- lm(D$GHT4~I(D$tau^1)+I(D$tau^2)+I(D$tau^4)-1)
LM3c <- LM3$coe; LM4c <- LM4$coe
Tau <- seq(0,1, by=.01); abline(0,0, lty=2, col=3)
lines(Tau, 0 + LM3c[1]*Tau^1 + LM3c[2]*Tau^2 + LM3c[3]*Tau^4, col=4, lwd=3)
lines(Tau, -(0 + LM4c[1]*Tau^1 + LM4c[2]*Tau^2 + LM4c[3]*Tau^4), col=2, lwd=3) #
## End(Not run)
## Not run:
# Let us compare the conditional simulation method of copBasic by numerics and by the
# above analytical solution for the Gumbel-Hougaard copula to two methods implemented
# by package gumbel, a presumed Archimedean technique by package acopula, and an
# Archimedean technique by package copula. Setting seeds by each "method" below does
# not appear diagnostic because of the differences in which the simulations are made.
nsim <- 10000; kn <- "kendall" # The theoretical KENDALL TAU is (1.5-1)/1.5 = 1/3
# Simulate by conditional simulation using numerical derivative and then inversion
A <- cor(copBasic::simCOP(nsim, cop=GHcop, para=1.5, graphics=FALSE), method=kn)[1,2]
U <- runif(nsim) # GHcop.derCOPinv() comes from earlier in this documentation.
V <- sapply(1:nsim, function(i) { GHcop.derCOPinv(U[i], runif(1), para=1.5) })
# Simulate by conditional simulation using exact analytical solution
B <- cor(U, y=V, method=kn); rm(U, V)
# Simulate by the "common frailty" technique
C <- cor(gumbel::rgumbel(nsim, 1.5, dim=2, method=1), method=kn)[1,2]
# Simulate by "K function" (Is the K function method, Archimedean?)
D <- cor(gumbel::rgumbel(nsim, 1.5, dim=2, method=2), method=kn)[1,2]
# Simulate by an Archimedean implementation (presumably)
E <- cor(acopula::rCopula(nsim, pars=1.5), method=kn)[1,2]
# Simulate by an Archimedean implementation
G <- cor(copula::rCopula(nsim, copula::gumbelCopula(1.5)), method=kn)[1,2]
K <- round(c(A, B, C, D, E, G), digits=5); rm(A, B, C, D, E, G, kn); tx <- ", "
message("Kendall Tau: ", K[1], tx, K[2], tx, K[3], tx, K[4], tx, K[5], tx, K[6])
# Kendall Tau: 0.32909, 0.32474, 0.33060, 0.32805, 0.32874, 0.33986 -- run 1
# Kendall Tau: 0.33357, 0.32748, 0.33563, 0.32913, 0.32732, 0.32416 -- run 2
# Kendall Tau: 0.34311, 0.33415, 0.33815, 0.33224, 0.32961, 0.33008 -- run 3
# Kendall Tau: 0.32830, 0.33573, 0.32756, 0.33401, 0.33567, 0.33182 -- nsim=50000!
# All solutions are near 1/3 and it is unknown without further study which of the
# six methods would result in the least bias and (or) sampling variability.
## End(Not run)
wasquith/copBasic documentation built on Dec. 13, 2024, 6:39 p.m.
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| GHcop | R Documentation |
The Gumbel–Hougaard Extreme Value Copula
Description
SYMMETRIC GUMBEL-HOUGAARD—The Gumbel–Hougaard copula (Nelsen, 2006, pp. 118 and 164) is
\mathbf{C}_{\Theta}(u,v) = \mathbf{GH}(u,v) = \mathrm{exp}\{-[(-\log u)^\Theta+(-\log v)^\Theta]^{1/\Theta}\}\mbox{,}
where \Theta \in [1 , \infty). The copula here is a bivariate extreme value copula (BEV). The parameter \Theta is readily estimated using a Kendall Tau (say a sample version \hat\tau) where the \tau of the copula (\tau_\mathbf{C}) is defined as
\tau_\mathbf{C} = \frac{\Theta - 1}{\Theta} \rightarrow \Theta = \frac{1}{1-\tau}\mbox{.}
The copula is readily extended into d dimensions by
\mathbf{C}_{\Theta}(u,v) = \mathrm{exp}\{-[(-\log u_1)^\Theta+\cdots+(-\log u_d)^\Theta]^{1/\Theta}\}\mbox{.}
However, such an implementation is not available in the copBasic package.
Every Gumbel–Hougaard copula is a multivariate extreme value (MEV) copula, and hence useful in analysis of extreme value distributions. The Gumbel–Hougaard copula is the only Archimedean MEV (Salvadori et al., 2007, p. 192). The Gumbel–Hougaard copula has respective lower- and upper-tail dependency parameters of \lambda^L = 0 and \lambda^U = 2 - 2^{1/\Theta}, respectively. Nelsen (2006, p. 96) shows that \mathbf{C}^r_\theta(u^{1/r}, v^{1/r}) = \mathbf{C}_\theta(u,v) so that every Gumbel–Hougaard copula has a property known as max-stable. A dependence measure uniquely defined for BEV copulas is shown under rhobevCOP.
A comparison through simulation between Gumbel–Hougaard implementations by the R packages acopula, copBasic, copula, and Gumbel is shown in the Examples section. At least three divergent techniques for random variate generation are used amongst those packages. The simulations also use copBasic-style random variate generation (conditional simulation) using an analytical-numerical hybrid solution to conditional inverse described in the Note section.
TWO-PARAMETER GUMBEL–HOUGAARD—A permutation symmetric (isCOP.permsym) but almost certainly radial asymmetric (isCOP.radsym) version of the copula is readily constructed (Brahimi et al., 2015) into a two-parameter version:
\mathbf{C}(u,v; \beta_1, \beta_2) =
\biggl[
\biggl(\bigl(u^{-\beta_2} -1\bigr)^{\beta_1} +
\bigl(v^{-\beta_2} -1\bigr)^{\beta_1}
\biggr)^{1/\beta_1} + 1
\biggr]^{-1/\beta_2}\mbox{,}
where \beta_1 \ge 1 and \beta_2 > 0. Both parameters controls the general level of association, whereas parameter \beta_2 can be thought of as controlling left-tail dependency (taildepCOP, \lambda^{[U\mid L]}_{(\beta_1, \beta_2)}; e.g.
\lambda^U_{(1.5; \beta_2)} = 0.413 for all \beta_2 but
\lambda^L_{(1.5; 0.2)} = 0.811 and
\lambda^L_{(1.5; 2.2)} = 0.099. Brahimi et al. (2015) report a Spearman Rho (rhoCOP) for a \mathbf{GH}_{(1.5, 0.2)}(u,v) is 0.5, which is readily confirmed in copBasic by the function call rhoCOP(cop=GHcop, para=c(1.5,0.2)). The two-parameter \mathbf{GH} is triggered if the length of the para argument is exactly 2.
ASYMMETRIC GUMBEL–HOUGAARD—An asymmetric version of the copula is readily constructed (Joe, 2014, p. 185–186) into a three-parameter version with Marshall–Olkin copulas on the boundaries:
\mathbf{C}(u,v; \Theta, \pi_2, \pi_3) = \mathrm{exp}[-\mathcal{A}(-\log u, -\log v; \Theta, \pi_2, \pi_3)]\mbox{,}
where \Theta \ge 1 as before, 0 \le \pi_2, \pi_3 \le 1, and
\mathcal{A}(x, y; \Theta, \pi_2, \pi_3) = [(\pi_2 x)^\Theta + (\pi_3 y)^\Theta]^{1/\Theta} + (1-\pi_2)x + (1-\pi_3)y\mbox{.}
The asymmetric \mathbf{GH} is triggered if the length of the para argument is exactly 3. The GHcop function provides no mechanism for estimation of the parameters for the asymmetric version. Reviewing simulations, the bounds on the \pi parameters in Joe (2014, p. 185) “[0 \le \pi_2 < \pi_3 \le 1]” might be incorrect—by Joe back referencing to Joe (2014, eq. 4.35, p. 183) the \pi-limits as stated for copBasic are shown. An algorithm for parameter estimation for the asymmetric \mathbf{GH} using two different measures of bivariate skewness as well as an arbitrary measure of association is shown in section Details in joeskewCOP.
Usage
GHcop(u, v, para=NULL, tau=NULL, tau.big=0.985, cor=NULL, ...)
Arguments
u |
Nonexceedance probability |
v |
Nonexceedance probability |
para |
A vector (single element or triplet) of parameters—the |
tau |
Kendall Tau |
tau.big |
The largest value for |
cor |
A copBasic syntax for “the correlation coefficient” suitable for the copula—a synonym for |
... |
Additional arguments to pass. |
Details
Numerical experiments seem to indicate for \tau_\mathbf{C} > 0.985 that failures in the numerical partial derivatives in derCOP and derCOP2 result—a \tau_\mathbf{C} this large is indeed large. As \Theta \rightarrow \infty the Gumbel–Hougaard copula becomes the Fréchet–Hoeffding upper-bound copula \mathbf{M} (see M). A \tau_\mathbf{C} \approx 0.985 yields \Theta \approx 66 + 2/3, then for \Theta > 1/(1-\tau_\mathbf{C}) flips over to the \mathbf{M} copula with a warning issued.
Value
Value(s) for the copula are returned using the \Theta as set by argument para. Alternative returned values are possible: (1) If para=NULL and tau is set, then \tau_\mathbf{C} \rightarrow \Theta and an R list is returned. (2) If para=NULL and tau=NULL, then an attempt to estimate \Theta from the u and v is made by \mathrm{cor}(u,v)_\tau \rightarrow \tau_\mathbf{C} \rightarrow \Theta by either trigger using cor(u,v, method="kendall") in R, and an R list is returned. The possibly returned list has the following elements:
para |
The computed |
tau |
The sample estimate of |
Note
SYMMETRIC GUMBEL–HOUGAARD—A function for the derivative of the copula (Joe, 2014, p. 172) given u is
"GHcop.derCOP" <- function(u, v, para=NULL, ...) {
x <- -log(u); y <- -log(v)
A <- exp(-(x^para + y^para)^(1/para)) * (1 + (y/x)^para)^(1/para - 1)
return(A/u)
}
that can be tested by the following
Theta <- 1/(1-.15) # a Kendall Tau of 0.15
GHcop.derCOP( 0.5, 0.75, para=Theta) # 0.7787597
derCOP(cop=GHcop, 0.5, 0.75, para=Theta) # 0.7787597
# The next two nearly return same value but conversion to GRVs
# (Gumbel Reduced Variates) to magnify the numerical differences.
# The GHcop.derCOP is expected to be the more accurate of the two.
lmomco::prob2grv(GHcop.derCOP( 0.5, 0.9999999, para=Theta)) # 18.83349
lmomco::prob2grv(derCOP(cop=GHcop, 0.5, 0.9999999, para=Theta)) # 18.71497
lmomco::prob2grv(derCOP(cop=GHcop, 0.5, 0.9999999, para=Theta,
delu=.Machine$double.eps^.25)) # 18.83341
where the last numerical approximation shows that tighter tolerance is needed. A function for the inverse of the derivative (Joe, 2014, p. 172) given u by an analytical-numerical hybrid is
"GHcop.derCOPinv" <- function(u,t, para=NULL, verbose=FALSE,
tol=.Machine$double.eps, ...) {
if(length(u) > 1) warning("only the first value of u will be used")
if(length(t) > 1) warning("only the first value of t will be used")
if(is.null(para)) { warning("para can not be NULL"); return(NA) }
u <- u[1]; t <- t[1]; rt <- NULL
x <- -log(u); A <- (x + (para - 1)*log(x) - log(t))
hz <- function(z) { z + (para - 1)*log(z) - A }
zmax <- x; i <- 0; hofz.lo <- hz(zmax)
if(sign(hofz.lo) != -1) warning("sign for h(z) is not negative!")
while(1) {
i <- i + 1
if(i > 100) {
warning("maximum iterations looking for zmax reached"); break
}
# increment zmax by 1/2 log cycle, sign(hofz.lo) should be negative!
if(sign(hz(zmax <- zmax + 1/2)) != sign(hofz.lo)) break
}
try(rt <- uniroot(hz, c(x, zmax), tol=tol, ...), silent=FALSE)
if(verbose) print(rt)
if(is.null(rt)) {
warning("NULL on the inversion of the GH copula derivative")
return(NA)
}
zo <- rt$root
y <- (zo^para - x^para)^(1/para)
names(y) <- NULL
return(exp(-y))
}
that can be tested by the following, which also shows how to increase the tolerance on the numerical implementation
u <- 0.999; p <- 0.999 GHcop.derCOPinv( u, p, para=1.56) # 0.999977 derCOPinv(cop=GHcop, u, p, para=1.56) # 1 (unity), needs tighter tolerance derCOPinv(cop=GHcop, u, p, para=1.56, tol=.Machine$double.eps/10) # 0.999977
ASYMMETRIC GUMBEL–HOUGAARD—Set \tau_\mathbf{C} = 0.35 then for a symmetric and then reflection on the 1:1 line of the asymmetric Gumbel–Hougaard copula and compute the primary parameter \Theta, and lastly, compute three bivariate \nu_\mathbf{C} skewnesses (nuskewCOP):
Theta1 <- uniroot(function(t) {
0.35 - tauCOP(cop=GHcop, para=c(t)) }, c(1,10))$root
Theta2 <- uniroot(function(t) { # asymmetric
0.35 - tauCOP(cop=GHcop, para=c(t, 0.6, 0.9)) }, c(1,30))$root
Theta3 <- uniroot(function(t) { # asymmetric reflection on 1:1
0.35 - tauCOP(cop=GHcop, para=c(t, 0.9, 0.6)) }, c(1,30))$root
# Theta1 = 1.538462 and Theta2 = Theta3 = 2.132856
# Three "skews" based on a combination of U, V, and C(u,v) [nuskew()]
nuskewCOP(cop=GHcop, 1.538462) # zero bivariate skewness
nuskewCOP(cop=GHcop, c(2.132856, 0.6, 0.9)) # 0.008245653
nuskewCOP(cop=GHcop, c(2.132856, 0.9, 0.6)) # -0.008245653
So, we see, holding \tau_\mathbf{C} constant, that the \mathbf{GH} has a \nu_{\mathbf{GH}(1.538)} = 0 but the asymmetric case \nu_{\mathbf{GH}(2.133, 0.6, 0.9)} = 0.0082 and \nu_{\mathbf{GH}(2.133, 0.9, 0.6)} = -0.0082 where the change in sign represents reflection about the 1:1 line. Finally, compute L-coskew by large simulation and the adjective “bow” representing the direction of bowing or curvature of the principle copula density.
# Because the Tau's are all similar, there is nothing to learn from the
# L-correlation, let us inspect the L-coskew instead:
coT3.1<-lmomco::lcomoms2(simCOP(n=8000, cop=GHcop, para=c(Theta1 )))$T3
coT3.2<-lmomco::lcomoms2(simCOP(n=8000, cop=GHcop, para=c(Theta2,.6,.9)))$T3
coT3.3<-lmomco::lcomoms2(simCOP(n=8000, cop=GHcop, para=c(Theta3,.9,.6)))$T3
# The simulations for Theta1 have no curvature about the diagonal.
# The simulations for Theta2 have curvature towards the upper left.
# The simulations for Theta3 have curvature towards the lower right.
message("# L-coskews: ",round(coT3.1[1,2],digits=4),"(symmetric) ",
round(coT3.2[1,2],digits=4),"(asym.--bow UL) ",
round(coT3.3[1,2],digits=4),"(asym.--bow UL)")
message("# L-coskews: ",round(coT3.1[2,1],digits=4),"(symmetric) ",
round(coT3.2[2,1],digits=4),"(asym.--bow LR) ",
round(coT3.3[2,1],digits=4),"(asym.--bow LR)")
# L-coskews: 0.0533(symmetric) 0.1055(asym.--bow UL) 0.0021(asym.--bow UL)
# L-coskews: 0.0679(symmetric) 0.0112(asym.--bow LR) 0.1154(asym.--bow LR)
Thus, the L-comoments (Asquith, 2011) using their sample values measure something fundamental about the bivariate association between the three copulas choosen. The L-coskews for the symmetrical case are about equal and are
\tau^{\mathbf{GH}(\Theta_1)}_{3[12]} \approx \tau^{\mathbf{GH}(\Theta_1)}_{3[21]} \rightarrow (0.0533 + 0.0679)/2 = 0.0606\mbox{,}
whereas the L-coskew for the curvature to the upper left are
\tau^{\mathbf{GH}(\Theta_2, 0.6, 0.9)}_{3[12]} = 0.1055\mbox{\ and\ } \tau^{\mathbf{GH}(\Theta_2, 0.6, 0.9)}_{3[12]} = 0.0112\mbox{,}
whereas the L-coskew for the curvature to the lower right are
\tau^{\mathbf{GH}(\Theta_3, 0.9, 0.6)}_{3[12]} = 0.0021\mbox{\ and\ } \tau^{\mathbf{GH}(\Theta_3, 0.6, 0.9)}_{3[12]} = 0.1154\mbox{.}
Thus, the \pi_2 and \pi_3 parameters as choosen add about ((0.1154+0.1055)/2 - 0.0606 \rightarrow 0.1105 - 0.0606 = 0.05) L-coskew units to the bivariate distribution.
Author(s)
W.H. Asquith
References
Asquith, W.H., 2011, Distributional analysis with L-moment statistics using the R environment for statistical computing: Createspace Independent Publishing Platform, ISBN 978–146350841–8.
Brahimi, B., Chebana, F., and Necir, A., 2015, Copula representation of bivariate L-moments—A new estimation method for multiparameter two-dimensional copula models: Statistics, v. 49, no. 3, pp. 497–521.
Joe, H., 2014, Dependence modeling with copulas: Boca Raton, CRC Press, 462 p.
Nelsen, R.B., 2006, An introduction to copulas: New York, Springer, 269 p.
Salvadori, G., De Michele, C., Kottegoda, N.T., and Rosso, R., 2007, Extremes in Nature—An approach using copulas: Springer, 289 p.
Zhang, L., and Singh, V.P., 2007, Gumbel–Hougaard copula for trivariate rainfall frequency analysis: Journal Hydrologic Engineering, v. 12, Special issue—Copulas in Hydrology, pp. 409–419.
See Also
M, GLcop, HRcop, tEVcop, rhobevCOP
Examples
Theta <- 2.2 # Let us see if numerical and analytical tail deps are the same.
del.lamU <- abs( taildepCOP(cop=GHcop, para=Theta)$lambdaU - (2-2^(1/Theta)) )
as.logical(del.lamU < 1E-6) # TRUE
## Not run:
# The simulations match Joe (2014, p. 72) for Gumbel-Hougaard
n <- 600; nsim <- 1000; set.seed(946) # see for reproducibility
SM <- sapply(1:nsim, function(i) { rs <- semicorCOP(cop=GHcop, para=1.35, n=n)
c(rs$botleft.semicor, rs$topright.semicor) })
RhoM <- round(mean(SM[1,]), digits=3)
RhoP <- round(mean(SM[2,]), digits=3)
SE.RhoM <- round( sd(SM[1,]), digits=3)
SE.RhoP <- round( sd(SM[2,]), digits=3)
SE.RhoMP <- round( sd(SM[2,] - SM[1,]), digits=3)
# Semi-correlations (sRho) and standard errors (SEs)
message("# sRho[-]=", RhoM, " (SE[-]=", SE.RhoM, ") Joe(p.72)=0.132 (SE[-]=0.08)")
message("# sRho[+]=", RhoP, " (SE[+]=", SE.RhoP, ") Joe(p.72)=0.415 (SE[+]=0.07)")
message("# SE(sRho[-] - sRho[+])=", SE.RhoMP, " Joe(p.72) SE=0.10")
# sRho[-]=0.134 (SE[-]=0.076) Joe(p.72)=0.132 (SE[-]=0.08)
# sRho[+]=0.407 (SE[+]=0.074) Joe(p.72)=0.415 (SE[+]=0.07)
# SE(sRho[-] - sRho[+])=0.107 Joe(p.72) SE=0.10
# Joe (2014, p. 72) reports the values 0.132, 0.415, 0.08, 0.07, 0.10, respectively.
## End(Not run)
## Not run:
file <- "Lcomom_study_of_GHcopPLACKETTcop.txt"
x <- data.frame(tau=NA, trho=NA, srho=NA, PLtheta=NA, PLT2=NA, PLT3=NA, PLT4=NA,
GHtheta=NA, GHT2=NA, GHT3=NA, GHT4=NA )
write.table(x, file=file, row.names=FALSE, quote=FALSE)
n <- 250 # Make a large number for very long CPU run but seems stable
for(tau in seq(0,0.98, by=0.005)) {
thetag <- GHcop(u=NULL, v=NULL, tau=tau)$para
trho <- rhoCOP(cop=GHcop, para=thetag)
GH <- simCOP(n=n, cop=GHcop, para=thetag, points=FALSE, ploton=FALSE)
srho <- cor(GH$U, GH$V, method="spearman")
thetap <- PLACKETTpar(rho=trho)
PL <- simCOP(n=n, cop=PLACKETTcop, para=thetap, points=FALSE, ploton=FALSE)
GHl <- lmomco::lcomoms2(GH, nmom=4); PLl <- lmomco::lcomoms2(PL, nmom=4)
x <- data.frame(tau=tau, trho=trho, srho=srho,
GHtheta=thetag, PLtheta=thetap,
GHT2=mean(c(GHl$T2[1,2], GHl$T2[2,1])),
GHT3=mean(c(GHl$T3[1,2], GHl$T3[2,1])),
GHT4=mean(c(GHl$T4[1,2], GHl$T4[2,1])),
PLT2=mean(c(PLl$T2[1,2], PLl$T2[2,1])),
PLT3=mean(c(PLl$T3[1,2], PLl$T3[2,1])),
PLT4=mean(c(PLl$T4[1,2], PLl$T4[2,1])) )
write.table(x, file=file, row.names=FALSE, col.names=FALSE, append=TRUE)
}
# After a processing run with very large "n", then meaningful results exist.
D <- read.table(file, header=TRUE); D <- D[complete.cases(D),]
plot(D$tau, D$GHT3, ylim=c(-0.08,0.08), type="n",
xlab="KENDALL TAU", ylab="L-COSKEW OR NEGATED L-COKURTOSIS")
points(D$tau, D$GHT3, col=2); points(D$tau, D$PLT3, col=1)
points(D$tau, -D$GHT4, col=4, pch=2); points(D$tau, -D$PLT4, col=1, pch=2)
LM3 <- lm(D$GHT3~I(D$tau^1)+I(D$tau^2)+I(D$tau^4)-1)
LM4 <- lm(D$GHT4~I(D$tau^1)+I(D$tau^2)+I(D$tau^4)-1)
LM3c <- LM3$coe; LM4c <- LM4$coe
Tau <- seq(0,1, by=.01); abline(0,0, lty=2, col=3)
lines(Tau, 0 + LM3c[1]*Tau^1 + LM3c[2]*Tau^2 + LM3c[3]*Tau^4, col=4, lwd=3)
lines(Tau, -(0 + LM4c[1]*Tau^1 + LM4c[2]*Tau^2 + LM4c[3]*Tau^4), col=2, lwd=3) #
## End(Not run)
## Not run:
# Let us compare the conditional simulation method of copBasic by numerics and by the
# above analytical solution for the Gumbel-Hougaard copula to two methods implemented
# by package gumbel, a presumed Archimedean technique by package acopula, and an
# Archimedean technique by package copula. Setting seeds by each "method" below does
# not appear diagnostic because of the differences in which the simulations are made.
nsim <- 10000; kn <- "kendall" # The theoretical KENDALL TAU is (1.5-1)/1.5 = 1/3
# Simulate by conditional simulation using numerical derivative and then inversion
A <- cor(copBasic::simCOP(nsim, cop=GHcop, para=1.5, graphics=FALSE), method=kn)[1,2]
U <- runif(nsim) # GHcop.derCOPinv() comes from earlier in this documentation.
V <- sapply(1:nsim, function(i) { GHcop.derCOPinv(U[i], runif(1), para=1.5) })
# Simulate by conditional simulation using exact analytical solution
B <- cor(U, y=V, method=kn); rm(U, V)
# Simulate by the "common frailty" technique
C <- cor(gumbel::rgumbel(nsim, 1.5, dim=2, method=1), method=kn)[1,2]
# Simulate by "K function" (Is the K function method, Archimedean?)
D <- cor(gumbel::rgumbel(nsim, 1.5, dim=2, method=2), method=kn)[1,2]
# Simulate by an Archimedean implementation (presumably)
E <- cor(acopula::rCopula(nsim, pars=1.5), method=kn)[1,2]
# Simulate by an Archimedean implementation
G <- cor(copula::rCopula(nsim, copula::gumbelCopula(1.5)), method=kn)[1,2]
K <- round(c(A, B, C, D, E, G), digits=5); rm(A, B, C, D, E, G, kn); tx <- ", "
message("Kendall Tau: ", K[1], tx, K[2], tx, K[3], tx, K[4], tx, K[5], tx, K[6])
# Kendall Tau: 0.32909, 0.32474, 0.33060, 0.32805, 0.32874, 0.33986 -- run 1
# Kendall Tau: 0.33357, 0.32748, 0.33563, 0.32913, 0.32732, 0.32416 -- run 2
# Kendall Tau: 0.34311, 0.33415, 0.33815, 0.33224, 0.32961, 0.33008 -- run 3
# Kendall Tau: 0.32830, 0.33573, 0.32756, 0.33401, 0.33567, 0.33182 -- nsim=50000!
# All solutions are near 1/3 and it is unknown without further study which of the
# six methods would result in the least bias and (or) sampling variability.
## End(Not run)
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