From Margin of Safety (Source):
Many investors insist on affixing exact values to their investments, seeking precision in an imprecise world, but business value cannot be precisely determined.
Reported book value, earnings, and cash flow are, after all, only the best guesses of accountants who follow a fairly strict set of standards and practices designed more to achieve conformity than to reflect economic value. Projected results are less precise still. You cannot appraise the value of your home to the nearest thousand dollars. Why would it be any easier to place a value on vast and complex businesses?
Not only is business value imprecisely knowable, it also changes over time, fluctuating with numerous macroeconomic, microeconomic, and market-related factors. So while investors at any given time cannot determine business value with precision, they must nevertheless almost continuously reassess their estimates of value in order to incorporate all known factors that could influence their appraisal.
Any attempt to value businesses with precision will yield values that are precisely inaccurate. The problem is that it is easy to confuse the capability to make precise forecasts with the ability to make accurate ones.Anyone with a simple, hand-held calculator can perform net present value (NPV) and internal rate of return (IRR) calculations. The advent of the computerized spreadsheet has exacerbated this problem, creating the illusion of extensive and thoughtful analysis, even for the most haphazard of efforts.
Typically, investors place a great deal of importance on the output, even though they pay little attention to the assumptions. "Garbage in, garbage out" is an apt description of the process.
In Security Analysis, Graham and David Dodd discussed the concept of a range of value – "The essential point is that security analysis does not seek to determine exactly what is the intrinsic value of a given security. It needs only to establish that the value is adequate – e.g., to protect a bond or to justify a stock purchase or else that the value is considerably higher or considerably lower than the market price. For such purposes an indefinite and approximate measure of the intrinsic value may be sufficient."
Indeed, Graham frequently performed a calculation known as net working capital per share, a back-of-the-envelope estimate of a company's liquidation value. His use of this rough approximation was a tacit admission that he was often unable to ascertain a company's value more precisely.