Sneak Preview: Admit It When It's Too Hard

In the case of Starbucks, the company reported a solid second quarter but disappointing July same-store sales on August 3, 2006. The stock, another one I'd been positive on, was down 8 percent by the close. Starbucks reported 4 percent same-store sales growth in July, when they'd given a pretty broad range of 3 to 7 percent growth. If they'd had 5.1 percent same-store sales growth, that would've been perceived as a beat, and the stock might have gone up.

But there's no way for you to know whether Starbucks will report 4 percent, or 5 percent, or 6 percent, so I don't want you trying to invest in any of these stocks on the basis of the same-store sales numbers. I know from experience that you can't count on them and you can't predict them reliably. It's one of those metrics that's just too darn hard.

There are countless other ways to make money investing. Stick with them and avoid the hard stuff. Howard Schultz, the great chairman of Starbucks, redeemed himself and his company the next month, but people got killed in the interim. Starbucks is worth sticking with as an investment because Schultz is so great and has a terrific long-term growth plan.

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Editor's note: This is one of Jim Cramer's 10 Rules from New Mistakes, New Rules: Ten Lessons From My Bad Calls, a special excerpt from his newest book, Jim Cramer's Mad Money: Watch TV, Get Rich, in stores now. Check back tomorrow for a new excerpt.