The median absolute deviation value of the Gumbel distribution



The Gumbel distribution is not only a useful model in the extreme value theory, but it’s also a nice example of a slightly right-skewed distribution (skewness \(\approx 1.14\)). Here is its density plot:


In some of my statistical experiments, I like to use the Gumbel distribution as a sample generator for hypothesis checking or unit tests. I also prefer the median absolute deviation (MAD) over the standard deviation as a measure of dispersion because it’s more robust in the case of non-parametric distributions. Numerical hypothesis verification often requires the exact value of the median absolute deviation of the original distribution. I didn’t find this value in the reference tables, so I decided to do another exercise and derive it myself. In this post, you will find a short derivation and the result (spoiler: the exact value is 0.767049251325708 * β). The general approach of the MAD derivation is common for most distributions, so it can be easily reused.

General approach

The median absolute deviation of a sample has a simple definition:

\[\mathcal{MAD}_0 = \textrm{Median}(|x_i - \textrm{Median}(x_i)|). \]

The exact formula for the distribution looks similar:

\[\mathcal{MAD}_0 = \textrm{Median}(|X - \textrm{Median}(X)|). \]

It’s convenient to use a scaled version of MAD in many practical cases to make it a consistent estimator for the standard deviation estimation. It’s often denoted as \(\mathcal{MAD}\) without any clarification. To avoid misinterpretation and highlight that we are working with a non-scaled version of MAD, we denote it as \(\mathcal{MAD}_0\) instead of just \(\mathcal{MAD}\).

We also denote the median distribution value as \(M = \textrm{Median}(X)\). Thus, the \(\mathcal{MAD}_0\) definition can be rewritten as

\[\mathcal{MAD}_0 = \textrm{Median}(|X - M|). \]

Since \(\mathcal{MAD}_0\) is the median of \(|X - M|\), we can conclude that the range \([M - \mathcal{MAD}_0; M + \mathcal{MAD}_0]\) contains \(50\%\) of the distribution:


This can be expressed using the distribution CDF (let’s call it \(F\)):

\[\begin{equation} \label{eq:main}\tag{1} F(M + \mathcal{MAD}_0) - F(M - \mathcal{MAD}_0) = 0.5. \end{equation} \]

Equation (\(\ref{eq:main}\)) is an important property of the median absolute deviation, which we will use to calculate it’s exact value.

Gumbel distribution

The Gumbel distribution is parametrized by \(\mu\) (location) and \(\beta\) (scale). For these parameters, the median value and the CDF are well-known (see [HoSM], 1.3.6.6.16):

\[M = \mu - \beta \cdot \ln(\ln(2)), \quad F(x) = e^{-e^{-(x-\mu)/\beta}}. \]

The \(\mathcal{MAD}_0\) value is directly proportional to the scale parameter \(\beta\), so let’s introduce an auxiliary variable \(p\) for the “descaled version” of \(\mathcal{MAD}_0\):

\[p = \mathcal{MAD}_0 / \beta. \]

Next, let’s express \(F(M + \mathcal{MAD}_0)\) via \(p\):

\[\begin{split} F(M + \mathcal{MAD}_0) & = F(\mu - \beta \cdot \ln(\ln(2)) + p \beta) = \\ & = e^{-e^{-(\mu - \beta \cdot \ln(\ln(2)) + p \beta - \mu)/\beta}} = \\ & = e^{-e^{\ln(\ln(2)) - p}} = e^{-\ln(2) e^{-p}} = 0.5^{e^{-p}}, \end{split} \]

In the same way, we can show that

\[F(M - \mathcal{MAD}_0) = 0.5^{e^{p}}. \]

Now, equation (\(\ref{eq:main}\)) can be transformed to

\[0.5^{e^{-p}} - 0.5^{e^{p}} = 0.5. \]

This equation can be solved numerically. Here is a short R script which calculates the result:

library(rootSolve)
options(digits = 15)
f <- function(p) 0.5^exp(-p) - 0.5^exp(p) - 0.5
uniroot.all(f, c(-10, 10), tol = 1e-15)

The numerical solution is \(p = 0.767049251325708 \). Since \(p = \mathcal{MAD}_0 / \beta\), the exact solution looks as follows:

\[\mathcal{MAD}_0 = 0.767049251325708 \beta. \]

Now we can use it in experiments (copy-pastable version: 0.767049251325708 * β).

Numerical simulation

Let’s double-check that our calculations are correct. Below you can see an R script that generates 1000 random samples from the Gumbel distribution with \(\mu = 0,\; \beta = 1\) (1000 elements in each sample). Next, it calculates the MAD estimation for each sample using the Harrell-Davis quantile estimator for the median estimations (see [Harrell1982]). After that, it calculates the median of all MAD estimations and prints the result.

library(evd)
library(Hmisc)

# Setting seed for rgumbel to achieve deterministic result
set.seed(38)
# Harrell-Davis-powered median
hdmedian <- function(x) as.numeric(hdquantile(x, 0.5))
# MAD value which is based on the Harrell-Davis-powered median
hdmad <- function(x) hdmedian(abs(x - hdmedian(x)))
# 1000 trails of MAD estimating for a sample from the Gubmel distribution
mads <- sapply(1:1000, function(i) hdmad(rgumbel(1000)))
# Print median of all trials
hdmedian(mads)

This script’s output is 0.7670284 which is close enough to the exact value 0.767049251325708.

Conclusion

The Gumbel distribution can be used as a good model of a slightly right-skewed distribution. If we are playing with an algorithm that involves MAD estimations (e.g., MAD-based outlier detector), we can check the precision of our calculations using the exact MAD value which is \(\mathcal{MAD}_0 = 0.767049251325708 \beta\).

The general approach can be reused for any other distributions with known median and CDF. You can get the exact \(\mathcal{MAD}_0\) value by solving the equation \(F(M + \mathcal{MAD}_0) - F(M - \mathcal{MAD}_0) = 0.5\) which is correct for the distribution of any form.

References



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