Both Charlie and I believe that Black-Scholes produces wildly inappropriate values when applied to long-dated options. We set out one absurd example in these pages two years ago. More tangibly, we put our money where our mouth was by entering into our equity put contracts. By doing so, we implicitly asserted that the Black-Scholes calculations used by our counterparties or their customers were faulty.
We continue, nevertheless, to use that formula in presenting our financial statements. Black-Scholes is the accepted standard for option valuation – almost all leading business schools teach it – and we would be accused of shoddy accounting if we deviated from it. Moreover, we would present our auditors with an insurmountable problem were we to do that: They have clients who are our counterparties and who use Black-Scholes values for the same contracts we hold. It would be impossible for our auditors to attest to the accuracy of both their values and ours were the two far apart.
Part of the appeal of Black-Scholes to auditors and regulators is that it produces a precise number. Charlie and I can't supply one of those. We believe the true liability of our contracts to be far lower than that calculated by Black-Scholes, but we can't come up with an exact figure – anymore than we can come up with a precise value for GEICO, BNSF, or for Berkshire Hathaway itself. Our inability to pinpoint a number doesn't bother us: We would rather be approximately right than precisely wrong.
John Kenneth Galbraith once slyly observed that economists were most economical with ideas: They made the ones learned in graduate school last a lifetime. University finance departments often behave similarly. Witness the tenacity with which almost all clung to the theory of efficient markets throughout the 1970s and 1980s, dismissively calling powerful facts that refuted it "anomalies." (I always love explanations of that kind: The Flat Earth Society probably views a ship's circling of the globe as an annoying, but inconsequential, anomaly.)
Academics' current practice of teaching Black-Scholes as revealed truth needs re-examination. For that matter, so does the academic's inclination to dwell on the valuation of options. You can be highly successful as an investor without having the slightest ability to value an option. What students should be learning is how to value a business. That's what investing is all about. - Warren Buffett
Here's a quote from Munger on the same subject from the 2003 Shareholder Meeting:
Black-Scholes is a know-nothing system. If you know nothing about value — only price — then Black-Scholes is a pretty good guess at what a 90-day option might be worth. But the minute you get into longer periods of time, it's crazy to get into Black-Scholes. For example, at Costco we issued stock options with strike prices of $30 and $60, and Black-Scholes valued the $60 ones higher. This is insane. - Charlie Munger
At the same meeting Buffett added the following:
Buffett: We like this kind of insanity. We will pay you real money if you deliver someone to our office who is willing to offer us three-year options that we can pick and choose from.
When I first understood how prevalent the use of Black-Scholes was to value long-dated options more than a decade ago I figured by now things would have at least changed marginally toward something more rational. Nope.
I've learned to never underestimate how sticky the entrenched use of a bad idea is.
Adam
Black-Scholes and the Flat Earth Society: 2010 Berkshire Shareholder Letter Highlights
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