Class: Statistics::Distribution::Poisson

Inherits:
Object
  • Object
show all
Defined in:
lib/statistics/distribution/poisson.rb

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(l) ⇒ Poisson

Returns a new instance of Poisson.



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# File 'lib/statistics/distribution/poisson.rb', line 9

def initialize(l)
  self.expected_number_of_occurrences = l
end

Instance Attribute Details

#expected_number_of_occurrences ⇒ Object Also known as: mean, variance

Returns the value of attribute expected_number_of_occurrences.



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# File 'lib/statistics/distribution/poisson.rb', line 4

def expected_number_of_occurrences
  @expected_number_of_occurrences
end

Instance Method Details

#cumulative_function(k) ⇒ Object 



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# File 'lib/statistics/distribution/poisson.rb', line 24

def cumulative_function(k)
  return if k < 0 || expected_number_of_occurrences < 0

  k = k.to_i

  upper = Math.lower_incomplete_gamma_function((k + 1).floor, expected_number_of_occurrences)
  lower = Math.factorial(k.floor)

  # We need the right tail, i.e.: The upper incomplete gamma function. This can be
  # achieved by doing a substraction between 1 and the lower incomplete gamma function.
  1 - (upper/lower.to_f)
end

#probability_mass_function(k) ⇒ Object 



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# File 'lib/statistics/distribution/poisson.rb', line 13

def probability_mass_function(k)
  return if k < 0 || expected_number_of_occurrences < 0

  k = k.to_i

  upper = (expected_number_of_occurrences ** k) * Math.exp(-expected_number_of_occurrences)
  lower = Math.factorial(k)

  upper/lower.to_f
end