When it comes to buying and selling their companies' stocks, there are two kinds of corporate insiders: those who trade on a fixed schedule and those who don't. Investors should be careful to distinguish between the two.

Microsoft chairman Bill Gates falls into the former camp, selling 20 million shares of the company he founded every three months. Those sales are made under a 10b5-1 plan, the Securities and Exchange rule that allows executives to set up stock transactions in advance and thus not run afoul of insider-trading restrictions. But Gates's sales are so regular that the fact that they are made under a 10b5-1 plan is besides the point.