# File lib/ai4r/neural_network/backpropagation.rb, line 224
      def calculate_output_deltas(expected_values)
        output_values = @activation_nodes.last
        output_deltas = []
        output_values.each_index do |output_index|
          error = expected_values[output_index] - output_values[output_index]
          output_deltas << @derivative_propagation_function.call(
            output_values[output_index]) * error
        end
        @deltas = [output_deltas]
      end