2x2 table chi squared test ... use at your own risk ;)

chisq_test.rb

require 'statistics2'

def chisq_test(a, b, c, d)
  total = a + b + c + d
  row1_total = a + b
  row2_total = c + d
  col1_total = a + c
  col2_total = b + d
  e_a = (row1_total * col1_total).to_f / total
  e_b = (row1_total * col2_total).to_f / total
  e_c = (row2_total * col1_total).to_f / total
  e_d = (row2_total * col2_total).to_f / total
  chisq =
    ((a - e_a)**2 / e_a) + ((b - e_b)**2 / e_b) + ((c - e_c)**2 / e_c) + ((d - e_d)**2 / e_d)
  Statistics2.chi2_x(1, chisq)
end

pp chisq_test(0,2,3,4)       # => 0.25683925795785645
pp chisq_test(1,2,3,4)       # => 0.7781596861761657
pp chisq_test(10,20,30,40)   # => 0.372998483613487
pp chisq_test(100,20,30,40)  # => 7.044113736753843e-09
pp chisq_test(1000,20,30,40) # => 0.0