In this Motley Fool classic, former Fool Randy Befumo explains the importance of knowing and abiding by your circle of competence. This piece originally ran on Oct. 10, 2000.

Valuation and quality form the length and width of the investment world. These two dimensions indicate, in the end, what the size the investment return will be. Like everything else in life, our ability to use these tools successfully has finite limits. In fact, our ability to measure valuation or quality is actually fairly limited, despite the seeming precision that the numbers offer us. Although we always have the option of leaving an extra "margin of safety" in a purchase, by being conservative in our assessments of the valuation and underlying quality to try to offset our human limitations, properly assessing another dimension of the investment process -- depth of knowledge -- is probably even more vital to avoiding mistakes.

Be like Buffett

Depth of knowledge is a straightforward concept, even though it's erratically employed. Simply put, it's a measure of how well you understand a company's economic model -- the way it interacts with customers, suppliers, distributors, and investors as part of its daily operations. True depth of knowledge is erratically employed because of most investors' capacity for self-delusion. This is particularly true among investors who attempt to expand their idiosyncratic experience as a customer beyond its logical limits, allowing this impressionistic vantage point to color their perception of the company's overall economic model.

Famously, Berkshire Hathaway chairman Warren Buffett refused to fall into this trap during the great bull market/tech bubble of the late '90s. While many others chased names like JDS Uniphase (NASDAQ:JDSU) and CMGI (NASDAQ:CMGI) , Buffett admitted he didn't know enough about technology to invest in the sector. And, of course, it turned out just fine for him to stay in his circle of competence and stick with names like Coca-Cola (NYSE:KO) and Washington Post (NYSE:WPO) .

Understanding a business can be as complicated as the business itself, mitigated by our own personal knowledge and experience. The basic manufacturing model is probably the simplest business going. A company makes something, distributes it to the customer through set channels, and collects money from the customer in return. Another fairly simple model is the retailer, which receives manufactured goods from other companies and sells them directly to consumers. Or you can mix it up and have the hybrid manufacturer that not only makes products, but also markets and sells them. With the exception of natural resources, financial, and real estate companies, almost all companies involve some combination of the manufacturing or retailing format.

Beyond the product

Digging down deeper than this simple understanding of how sales dollars are converted into cash is essential. Who are the company's customers? Who are the company's suppliers? What factors affect end-user demand for the company's products or services? What is management's strategy for growing the business and creating shareholder value (not always one and the same)? The maxim "Buy what you know" is insufficient -- it should really be "Buy what you understand."

Heraclitus probably said it best when he quipped, "Much learning teaches little understanding." Actual concrete understanding a business is fairly rare. More often than not, it degenerates into a superficial sense of what the products are and how exciting they might be, or an idiosyncratic consumer familiarity with the product inflated into a purported understanding of the company's economic model.

Understanding goes beyond just reading the SEC filings. Routine, boilerplate warnings about potential pricing irrationality in a market just don't have the same power as the concrete and real experience of seeing the price for hard disk drives dive in the spot market. Many times, true understanding comes from months, or even years, of following a company before making your purchase. The first few times you look at a company's financial statements over the course of a year or two might be better conceived of as a "getting to know you" sort of engagement, rather than a "hey, let's go out" kind of thing. While just twiddling around with preliminaries can be boring, many investors will tell you that their best investments were not necessarily their first investments, but rather the companies they bought after years of watching them closely.

The power of patience

For me personally, this is true of my early 1996 purchase of America Online (now owned by Time Warner (NYSE:TWX) ) and my early 1997 purchase of Dell Computer (NASDAQ:DELL) . In fact, many expressed surprise that I hadn't owned either company until then, given the amount I'd written about both. The simple reality is that while I had an idea that Dell might be cheap way back in the first quarter of 1996, after it had been pounded by 33%, it took months for me to really understand the mechanics of asset management, and how Dell had used these to juice up its return on invested capital.

While you can argue that buying the stock on the first inclination that it might be cheap would have been better in that situation, I can think of a dozen other situations in which having patience and trying to learn more about a company through trade magazines, analyst reports, and a variety of other online and offline sources would have created a big enough picture to make buying as risk-free as possible from an information standpoint.

There's no set number that measures your knowledge of a company, which is probably the main reason why many investors fail to consider this important variable before investing. While you can assess the relative strength of a stock by glancing at a copy of Investor's Business Daily, assessing your own knowledge level is a much more introspective, difficult, and sometimes brutally honest process. However psychologically painful this self-assessment might be, if you really think about it, probably 50% of your major investment boo-boos come down to knowledge risk, with the remainder evenly split between valuation (buying too high) or quality (buying a lousy company).

Three cornerstones

While knowledge is important, it shouldn't be taken out of context with valuation and intrinsic business quality. The appearance of detailed knowledge about a company without a genuine understanding of how much cash the company will generate in the future can be as catastrophic an investment scenario as looking at valuation only and ignoring the company's underlying economic model.

Depth of knowledge, combined with low valuation and high quality, makes for outstanding investments. Any deviation from these three dimensions involves taking on the distasteful risk of losing money. This is not the end, however. When you combine this fully functional, three-dimensional investment model with an appropriate philosophical framework, then add the concept of time, you can engage in full-fledged security analysis.

Further Foolishness:

Coca-Cola and Dell are Motley Fool Inside Value picks, while Time Warner and Dell are Motley Fool Stock Advisor selections. Take the newsletter that best fits your investing style for a 30-day free trial.

Former Fool Randy Befumo is senior vice president, co-director of research at Legg Mason Capital Management. At the time of publication, Randy did not own shares of any company mentioned in this article. The Fool is investors helping investors.