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module Distribution
module Binomial
module Ruby_
class << self
def pdf(k, n, pr)
fail 'k>n' if k > n
Math.binomial_coefficient(n, k) * (pr**k) * (1 - pr)**(n - k)
end
# TODO: Use exact_regularized_beta for
# small values and regularized_beta for bigger ones.
def cdf(k, n, pr)
# (0..x.floor).inject(0) {|ac,i| ac+pdf(i,n,pr)}
Math.regularized_beta(1 - pr, n - k, k + 1)
end
def exact_cdf(k, n, pr)
out = (0..k).inject(0) { |ac, i| ac + pdf(i, n, pr) }
out = 1 if out > 1.0
out
end
def p_value(prob, n, pr)
ac = 0
(0..n).each do |i|
ac += pdf(i, n, pr)
return i if prob <= ac
end
end
alias_method :exact_pdf, :pdf
end
end
end
end
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