## Generalized Pareto Distribution

### Definition

The probability density function for the generalized Pareto distribution with
shape parameter *k* ≠ *0*, scale parameter
*σ*, and threshold parameter *θ*, is

$$y\text{}\text{}\text{\hspace{0.17em}}\text{\hspace{0.17em}}=\text{}\text{\hspace{0.17em}}f(x|k,\sigma ,\theta )=\text{}\text{}\text{}\text{}\text{}\text{}\text{\hspace{0.17em}}\left(\frac{1}{\sigma}\right){\left(1+k\frac{(x-\theta )}{\sigma}\right)}^{-1-\frac{1}{k}}$$

for *θ* < *x*, when *k* >
0, or for *θ* < *x* < *θ* –
*σ*/*k* when *k* <
0.

For *k* = 0, the density is

$$y\text{}\text{}\text{\hspace{0.17em}}\text{\hspace{0.17em}}=\text{}\text{\hspace{0.17em}}f(x|0\text{},\sigma ,\theta )=\text{}\text{}\text{}\text{}\text{}\text{}\text{\hspace{0.17em}}\left(\frac{1}{\sigma}\right){e}^{-\frac{(x-\theta )}{\sigma}}$$

for *θ* < *x*.

If *k* = 0 and *θ* = 0, the generalized Pareto
distribution is equivalent to the exponential distribution. If *k*
> 0 and *θ* = *σ*/*k*, the
generalized Pareto distribution is equivalent to the Pareto distribution with a
scale parameter equal to *σ*/*k* and a shape parameter equal to 1/*k*.

### Background

Like the exponential distribution, the generalized Pareto distribution is often
used to model the tails of another distribution. For example, you might have washers
from a manufacturing process. If random influences in the process lead to
differences in the sizes of the washers, a standard probability distribution, such
as the normal, could be used to model those sizes. However, while the normal
distribution might be a good model near its mode, it might not be a good fit to real
data in the tails and a more complex model might be needed to describe the full
range of the data. On the other hand, only recording the sizes of washers larger (or
smaller) than a certain threshold means you can fit a separate model to those tail
data, which are known as *exceedances*. You can use the
generalized Pareto distribution in this way, to provide a good fit to extremes of
complicated data.

The generalized Pareto distribution allows a continuous range of possible shapes that includes both the exponential and Pareto distributions as special cases. You can use either of those distributions to model a particular dataset of exceedances. The generalized Pareto distribution allows you to “let the data decide” which distribution is appropriate.

The generalized Pareto distribution has three basic forms, each corresponding to a limiting distribution of exceedance data from a different class of underlying distributions.

Distributions whose tails decrease exponentially, such as the normal, lead to a generalized Pareto shape parameter of zero.

Distributions whose tails decrease as a polynomial, such as Student's

*t*, lead to a positive shape parameter.Distributions whose tails are finite, such as the beta, lead to a negative shape parameter.

The generalized Pareto distribution is used in the tails of distribution fit
objects of the `paretotails`

object.

### Parameters

#### Fit Generalized Pareto Distribution

Generate a large number of random values from a Student's *t* distribution with 5 degrees of freedom, and then discard everything less than 2. Fit a generalized Pareto distribution to those exceedances.

rng("default") % For reproducibility t = trnd(5,5000,1); y = t(t > 2) - 2; paramEsts = gpfit(y)

`paramEsts = `*1×2*
0.1445 0.7225

Notice that the shape parameter estimate (the first element) is positive, which is what you would expect based on exceedances from a Student's *t* distribution.

```
h = histogram(y+2,2:0.5:12);
h.FaceColor = [0.8 0.8 1];
xgrid = linspace(2,12,1000);
line(xgrid,0.5*length(y)* ...
gppdf(xgrid,paramEsts(1),paramEsts(2),2));
xlim([2 12])
```

### Examples

#### Compute Generalized Pareto Distribution pdf

Compute the pdf of three generalized Pareto distributions. The first has shape parameter `k = -0.25`

, the second has `k = 0`

, and the third has `k = 1`

.

x = linspace(0,10,1000); y1 = gppdf(x,-.25,1,0); y2 = gppdf(x,0,1,0); y3 = gppdf(x,1,1,0);

Plot the three pdfs on the same figure.

figure; plot(x,y1,'-', x,y2,'--', x,y3,':') legend({'K < 0' 'K = 0' 'K > 0'});

## References

[1] Embrechts, P., C. Klüppelberg, and T. Mikosch. *Modelling
Extremal Events for Insurance and Finance*. New York: Springer,
1997.

[2] Kotz, S., and S. Nadarajah. *Extreme Value Distributions: Theory
and Applications*. London: Imperial College Press,
2000.