For a couple of reasons. In some ways, this would be nicer, do expected cases, it's going to tell you on average how much you expect to take, but it tends to be hard to compute, because to compute that, you have to know a distribution on input.

关注最快的情况，在某种意义上来说，因为一些原因这样想挺不错的，当我们处理一个给定的问题,计算平均时间的时候,是很难计算的，因为你并不知道输入的分布情况,这些输入会是怎么样的呢？

麻省理工公开课 - 计算机科学及编程导论课程节选

If you will buy stock in an electrical utility which has a guaranteed revenue stream, because no matter how bad the economy gets, a lot of people still use electricity, you don't expect it to fluctuate a lot.

如果你在有着,保障性年收入的电气供应股票投资，因为不管经济变得怎么查，人们还得用电，它不会降多少。

麻省理工公开课 - 计算机科学及编程导论课程节选

Because it is heavy you expect it is radioactive.

因为它的重，所以你想它应该是放射性元素。

麻省理工公开课 - 固态化学导论课程节选

Now you keep on trying and because the reinforcement is intermittent you don't expect it as much and so your behavior will persist across often a huge amount of time.

你会不停的尝试,因为此时的强化是间隔强化,你并不会抱有很高的期望,因此，你的行为,通常会持续相当长的一段时间。

耶鲁公开课 - 心理学导论课程节选