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Your best estimates for the future will not match up to the actual numbers for several reasons. First, even if your information sources are impeccable, you must convert raw information into forecasts, and any mistakes that you make at this stage will cause estimation error. Next, the path that you envision for a firm can prove to be hopelessly off. The firm may do much better or much worse than you expected it to perform, and the resulting earnings and cash flows will be different from your estimates; consider this firm-specific uncertainty. When valuing Cisco in 2001, for instance, we seriously underestimated how difficult it would be for the company to maintain its acquisition-driven growth in the future, and we overvalued the company as a consequence. Finally, even if a firm evolves exactly the way you expected it to, the macroeconomic environment can change in unpredictable ways. Interest rates can go up or down, and the economy can do much better or worse than expected. Our valuation of Marriott from November 2019 looks hopelessly optimistic, in hindsight, because we did not foresee the global pandemic in 2020 and the economic consequences for the hospitality business.
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Aswath Damodaran (The Little Book of Valuation: How to Value a Company, Pick a Stock, and Profit (Little Books. Big Profits))