# File lib/ai4r/neural_network/backpropagation.rb, line 269
      def calculate_error(expected_output)
        output_values = @activation_nodes.last
        error = 0.0
        expected_output.each_index do |output_index|
          error += 
            0.5*(output_values[output_index]-expected_output[output_index])**2
        end
        return error
      end