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module Statistics
module Distribution
class Binomial
attr_accessor :number_of_trials, :probability_per_trial
def initialize(n, p)
self.number_of_trials = n.to_i
self.probability_per_trial = p
end
def probability_mass_function(k)
return if k < 0 || k > number_of_trials
k = k.to_i
Math.combination(number_of_trials, k) *
(probability_per_trial ** k) * ((1 - probability_per_trial) ** (number_of_trials - k))
end
def cumulative_function(k)
return if k < 0 || k > number_of_trials
k = k.to_i
p = 1 - probability_per_trial
Math.incomplete_beta_function(p, number_of_trials - k, 1 + k)
end
def mean
number_of_trials * probability_per_trial
end
def variance
mean * (1 - probability_per_trial)
end
def mode
test = (number_of_trials + 1) * probability_per_trial
returned = if test == 0 || (test % 1 != 0)
test.floor
elsif (test % 1 == 0) && (test >= 1 && test <= number_of_trials)
[test, test - 1]
elsif test == number_of_trials + 1
number_of_trials
end
returned
end
end
end
end
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