# Maximum likelihood estimates for multivariate distributions

September 22, 2012
By

(This article was first published on Freakonometrics » R-english, and kindly contributed to R-bloggers)

Consider our loss-ALAE dataset, and – as in Frees & Valdez (1998) – let us fit a parametric model, in order to price a reinsurance treaty. The dataset is the following,

```> library(evd)
> data(lossalae)
> Z=lossalae
> X=Z[,1];Y=Z[,2]```

The first step can be to estimate marginal distributions, independently. Here, we consider lognormal distributions for both components,

```> Fempx=function(x) mean(X<=x)
> Fx=Vectorize(Fempx)
> u=exp(seq(2,15,by=.05))
> plot(u,Fx(u),log="x",type="l",
+ xlab="loss (log scale)")
> Lx=function(px) -sum(log(Vectorize(dlnorm)(
+ X,px[1],px[2])))
> opx=optim(c(1,5),fn=Lx)
> opx\$par
[1] 9.373679 1.637499
> lines(u,Vectorize(plnorm)(u,opx\$par[1],
+ opx\$par[2]),col="red")```

The fit here is quite good,

For the second component, we do the same,

```> Fempy=function(x) mean(Y<=x)
> Fy=Vectorize(Fempy)
> u=exp(seq(2,15,by=.05))
> plot(u,Fy(u),log="x",type="l",
+ xlab="ALAE (log scale)")
> Ly=function(px) -sum(log(Vectorize(dlnorm)(
+ Y,px[1],px[2])))
> opy=optim(c(1.5,10),fn=Ly)
> opy\$par
[1] 8.522452 1.429645
> lines(u,Vectorize(plnorm)(u,opy\$par[1],
+ opy\$par[2]),col="blue")```

It is not as good as the fit obtained on losses, but it is not that bad,

Now, consider a multivariate model, with Gumbel copula. We’ve seen before that it worked well. But this time, consider the maximum likelihood estimator globally.

```> Cop=function(u,v,a) exp(-((-log(u))^a+
+ (-log(v))^a)^(1/a))
> phi=function(t,a) (-log(t))^a
> cop=function(u,v,a) Cop(u,v,a)*(phi(u,a)+
+ phi(v,a))^(1/a-2)*(
+ a-1+(phi(u,a)+phi(v,a))^(1/a))*(phi(u,a-1)*
+ phi(v,a-1))/(u*v)
> L=function(p) {-sum(log(Vectorize(dlnorm)(
+ X,p[1],p[2])))-
+ sum(log(Vectorize(dlnorm)(Y,p[3],p[4])))-
+ sum(log(Vectorize(cop)(plnorm(X,p[1],p[2]),
+ plnorm(Y,p[3],p[4]),p[5])))}
> opz=optim(c(1.5,10,1.5,10,1.5),fn=L)
> opz\$par
[1] 9.377219 1.671410 8.524221 1.428552 1.468238```

Marginal parameters are (slightly) different from the one obtained independently,

```> c(opx\$par,opy\$par)
[1] 9.373679 1.637499 8.522452 1.429645
> opz\$par[1:4]
[1] 9.377219 1.671410 8.524221 1.428552```

And the parameter of Gumbel copula is close to the one obtained with heuristic methods in class.

Now that we have a model, let us play with it, to price a reinsurance treaty. But first, let us see how to generate Gumbel copula… One idea can be to use the frailty approach, based on a stable frailty. And we can use Chambers et al (1976)to generate a stable distribution. So here is the algorithm to generate samples from Gumbel copula

```> alpha=opz\$par[5]
> invphi=function(t,a) exp(-t^(1/a))
> n=500
> x=matrix(rexp(2*n),n,2)
> angle=runif(n,0,pi)
> E=rexp(n)
> beta=1/alpha
> stable=sin((1-beta)*angle)^((1-beta)/beta)*
+ (sin(beta*angle))/(sin(angle))^(1/beta)/
+ (E^(alpha-1))
> U=invphi(x/stable,alpha)
> plot(U)```

Here, we consider only 500 simulations,

Based on that copula simulation, we can then use marginal transformations to generate a pair, losses and allocated expenses,

```> Xloss=qlnorm(U[,1],opz\$par[1],opz\$par[2])
> Xalae=qlnorm(U[,2],opz\$par[3],opz\$par[4])```

In standard reinsurance treaties – see e.g. Clarke (1996) – allocated expenses are splited prorata capita between the insurance company, and the reinsurer. If $X$ denotes losses, and $Y$ the allocated expenses, a standard excess treaty can be has payoff

$\begin{cases} 0\cdot \boldsymbol{1}(X\leq R)\\ \displaystyle{\left(X-R+ \frac{X-R}{X}\cdot Y\right) \right )}\cdot\boldsymbol{1}(R
where $L$ denotes the (upper) limit, and $R$ the insurer’s retention. Using monte carlo simulation, it is then possible to estimate the pure premium of such a reinsurance treaty.

```> L=100000
> R=50000
> Z=((Xloss-R)+(Xloss-R)/Xloss*Xalae)*
+ (R<=Xloss)*(Xloss<L)+
+ ((L-R)+(L-R)/R*Xalae)*(L<=Xloss)
> mean(Z)
[1] 12596.45```

Now, play with it… it is possible to find a better fit, I guess…

### Arthur Charpentier

Arthur Charpentier, professor at UQaM in Actuarial Science. Former professor-assistant at ENSAE Paristech, associate professor at Ecole Polytechnique and assistant professor in Economics at Université de Rennes 1. Graduated from ENSAE, Master in Mathematical Economics (Paris Dauphine), PhD in Mathematics (KU Leuven), and Fellow of the French Institute of Actuaries.

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