没那么简单，也没那么复杂，

模型可以简单(当然也不像你说的那么简单），假设却可以复杂（不是一拍脑袋出来的）。

If the assumptions are garbages, the results will have no choice but go with it! 

Hence, to make the more accurate assumptions requires knowledge which takes years of experience....I don't have the ability to summarize in a few sentences;)