If you buy a stock because it has a great earnings report coming out and you expect the stock to rally when this occurs, then you should be ready to sell if that report comes out and the stock does nothing. If you bought a stock because its relative strength chart gave a buy signal, then be ready to liquidate your position if a relative strength sell signal is given. This also points out that it is extremely important to not only know when to buy a stock but also when to sell it. This brings to mind the basketball player shooting free throws. In other words, he will be successful in making both free throws less than half the time.
Investors have the same problem.
You must buy the stock right and you must sell the stock right. If you have ever bought a stock and watched it go up, then go right back down again, you know what I mean. You scored on the first shot but missed on the second shot. Both are important. Knowing why you purchased a stock can help you when it comes time to decide when to sell.