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Contents
module Statistics
module Distribution
class NegativeBinomial
attr_accessor :number_of_failures, :probability_per_trial
def initialize(r, p)
self.number_of_failures = r.to_i
self.probability_per_trial = p
end
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
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
def mean
(probability_per_trial * number_of_failures)/(1 - probability_per_trial).to_f
end
def variance
(probability_per_trial * number_of_failures)/((1 - probability_per_trial) ** 2).to_f
end
def skewness
(1 + probability_per_trial).to_f / Math.sqrt(probability_per_trial * number_of_failures)
end
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
end
end
end
Version data entries
4 entries across 4 versions & 1 rubygems