Method: RubyStatistics::Distribution::Poisson#cumulative_function

Defined in:
lib/ruby-statistics/distribution/poisson.rb

#cumulative_function(k) ⇒ Object 



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# File 'lib/ruby-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_r)
end