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module Distribution
module Binomial
module Ruby_
class << self
def pdf(k,n,pr)
raise "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)
(0..k).inject(0) {|ac,i| ac+pdf(i,n,pr)}
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 :exact_pdf :pdf
end
end
end
end
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