Class: Statistics::Distribution::NegativeBinomial

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

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(r, p) ⇒ NegativeBinomial

Returns a new instance of NegativeBinomial.



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# File 'lib/statistics/distribution/negative_binomial.rb', line 6

def initialize(r, p)
  self.number_of_failures = r.to_i
  self.probability_per_trial = p
end

Instance Attribute Details

#number_of_failures ⇒ Object

Returns the value of attribute number_of_failures.



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

def number_of_failures
  @number_of_failures
end

#probability_per_trial ⇒ Object

Returns the value of attribute probability_per_trial.



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

def probability_per_trial
  @probability_per_trial
end

Instance Method Details

#cumulative_function(k) ⇒ Object 



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# File 'lib/statistics/distribution/negative_binomial.rb', line 20

def cumulative_function(k)
  return if k < 0 || k > number_of_failures
  k = k.to_i

  1.0 - Math.incomplete_beta_function(probability_per_trial, k + 1, number_of_failures)
end

#mean ⇒ Object 



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# File 'lib/statistics/distribution/negative_binomial.rb', line 27

def mean
  (probability_per_trial * number_of_failures)/(1 - probability_per_trial).to_f
end

#mode ⇒ Object 



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# File 'lib/statistics/distribution/negative_binomial.rb', line 39

def mode
  if number_of_failures > 1
    up = probability_per_trial * (number_of_failures - 1)
    down = (1 - probability_per_trial).to_f

    (up/down).floor
  elsif number_of_failures <= 1
    0.0
  end
end

#probability_mass_function(k) ⇒ Object 



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# File 'lib/statistics/distribution/negative_binomial.rb', line 11

def probability_mass_function(k)
  return if number_of_failures < 0 || k < 0 || k > number_of_failures

  left = Math.combination(k + number_of_failures - 1, k)
  right = ((1 - probability_per_trial) ** number_of_failures) * (probability_per_trial ** k)

  left * right
end

#skewness ⇒ Object 



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# File 'lib/statistics/distribution/negative_binomial.rb', line 35

def skewness
  (1 + probability_per_trial).to_f / Math.sqrt(probability_per_trial * number_of_failures)
end

#variance ⇒ Object 



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# File 'lib/statistics/distribution/negative_binomial.rb', line 31

def variance
  (probability_per_trial * number_of_failures)/((1 - probability_per_trial) ** 2).to_f
end