def mean_squared_error
      return @mean_squared_error if @mean_squared_error

      numerator = errors.drop(@seasonality_strategy.start_index).map {|i| i ** 2 }.inject(&:+)
      @mean_squared_error = numerator / (errors.size - @seasonality_strategy.start_index).to_f
    end
def mean_squared_error
      return @mean_squared_error if @mean_squared_error

      numerator = errors.drop(@seasonality_strategy.start_index).map {|i| i ** 2 }.inject(&:+)
      @mean_squared_error = numerator / (errors.size - @seasonality_strategy.start_index).to_f
    end
Returns the mean squared error of the forecast.