def get_mean_value(predicted_values):
    """
    Method for calculate mean prediction value by all predicted value

    :param predicted_values: all predicted values
    :type predicted_values: list
    :return: mean prediction value
    :rtype: float
    """

    if len(predicted_values) == 0:
        return None

    sum_value = 0
    for value in predicted_values:
        sum_value += value

    return sum_value / len(predicted_values)