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经常看到大千的朋友Portfolio 只有可数的几个个股，很为他们捏一把汗。
有人会说，你是老几？ YMYD, 一边凉快去。
作为一个从业多年的工程师，我们知道设计工程系统比较重要的事情是“Mitigate Failures". 如果你的设计大部分时间工作，有些case 会导致人员伤亡，这样的产品总有一天会让公司破产。
同样的道理可以用于你的Portfolio.
不说最近的，当年Enron破产时，好些他的员工退休金统统没了。
扯远了。
这里有个文章介绍Risk/Reward计算，可以用于个股投资的评价。
其中关键因素是“止损”。通过科学的止损，可以达到减低Risk, 进而让你的投资Risk/Reward 达到比较统一的，可以接受的水平。这里必须提到，专业人士不是吃素的。他们要做大量的研究，确认一个投资不会让公司资金受到巨大损失。
所以，买股票之前，问一下，我的收益可能是多少，可能的损失是多大，值得冒这个险吗？
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https://www.investopedia.com/terms/r/riskrewardratio.asp
Consider this example: A trader purchases 100 shares of XYZ Company at $20 and places a stop-loss order at $15 to ensure that losses will not exceed $500. Also, assume that this trader believes that the price of XYZ will reach $30 in the next few months. In this case, the trader is willing to risk $5 per share to make an expected return of $10 per share after closing the position. Since the trader stands to make double the amount that they have risked, they would be said to have a 1:2 risk/reward ratio on that particular trade. Derivatives contracts such as put contracts, which give their owners the right to sell the underlying asset at a specified price, can be used to similar effect.
If an investor prefers to seek a 1:5 risk/reward ratio for a specified investment (five units of expected return for each additional unit of risk), then they can modify the stop-loss order and thus adjust the risk/reward ratio. But it is important to understand that by doing so the investors has changed the probability of success in their trade.
In the trading example noted above, suppose an investor set a stop-loss order at $18, instead of $15, and they continued to target a $30 profit-taking exit. By doing so they would certainly reduce the size of the potential loss (assuming no change to the number of shares), but they will have increased the likelihood that the price action will trigger their stop loss order. That's because the stop order is proportionally much closer to the entry than the target price is. So although the investor may stand to make a proportionally larger gain (compared to the potential loss), they have a lower probability of receiving this outcome.
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