Probability Distribution for the kth Greatest of a Sequence of n Random Numbers

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This Demonstration shows a simple example of using extreme value theory to calculate the probability density function for the greatest number in a series of
random numbers drawn from three distributions of some importance in financial calculations. Many observations of the
greatest number in a sample of size
drawn randomly from the specified distribution are taken and displayed via a histogram. The red curve is the graph of the analytical expression for the
smallest number, derived by considering the probability that, in a sequence of
random numbers,
numbers are greater than
and
numbers are smaller, all this weighted by the binomial distribution (see Details).
Contributed by: Felipe Dimer de Oliveira (March 2011)
Open content licensed under CC BY-NC-SA
Snapshots
Details
The probability that random numbers are smaller than
is given by the joint probability that each one of the random numbers is smaller than
. If the observations in the sample are independent of each other we then have
,
where is the cumulative distribution function (CDF). The probability density function of the maximum of the
observations is the derivative of
. The generalization for the
greatest number in a sequence is then given by the probability of obtaining
in
observations while obtaining
observations which are greater than
. To account for all possible permutations with which
can be less than
the probability must be weighted by a binomial factor,
.
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