Michael Mauboussin: Ruminations Of Risk Beta Versus Margin Of Safety

Get The Timeless Reading eBook in PDF

Get the entire 10-part series on Timeless Reading in PDF. Save it to your desktop, read it on your tablet, or email to your colleagues.

We respect your email privacy

Michael Mauboussin is the author of The Success Equation: Untangling Skill and Luck in Business, Sports, and Investing (Harvard Business Review Press, 2012), Think Twice: Harnessing the Power of Counterintuition (Harvard Business Press, 2009) and More Than You Know: Finding Financial Wisdom in Unconventional Places-Updated and Expanded (New York: Columbia Business School Publishing, 2008). More Than You Know was named one of “The 100 Best Business Books of All Time” by 800-CEO-READ, one of the best business books by BusinessWeek (2006) and best economics book by Strategy+Business (2006). He is also co-author, with Alfred Rappaport, of Expectations Investing: Reading Stock Prices for Better Returns (Harvard Business School Press, 2001).

Visit his site at: michaelmauboussin.com/

Michael Mauboussin: Ruminations Of Risk Beta Versus Margin Of Safety - Executive Summary

The concept of risk plays a central role in the investment process. Yet risk is an elusive concept - difficult to define, quantify and integrate.

In this report, we consider two senses of the term risk. The first is what is mainstream in finance circles: you can measure risk by seeing how much a stock bounces around versus “the market.” You can quantify this measure of risk through variance. The higher the variance-the larger the swings in relative price-the riskier the stock.

The second sense is the “margin of safety”, or a discount to expected value. The idea here is that for every stock there is an intrinsic value, and that the deeper the discount the stock price is to intrinsic value, the lower the risk.

We start this report by noting Warren Buffett’s attack on the first risk definition. (We suggest that he believes in, and acts according to, the second definition.) Unfortunately, Buffett attacks an idea that does not follow from finance theory. Without defending the traditional theory, we note that Buffett’s comments better reflect the second risk definition.

The conclusions from the report are as follows:

  • Volatility remains a reasonable measure of risk for the short-term. Long-term investors are better off considering the concept of margin of safety. There is evidence that suggests that volatility understates risk for up to four years, but overstates risk for holding periods beyond four years.
  • Margin of safety can be restated as a discount to expected value. Expected value is a function of the weighted probability of potential outcomes. Judgments on both outcomes and probabilities are tricky, but essential to the investment process.
  • Investors should base the magnitude of their investments on the size of the margin of safety. For companies with variable outcomes, the consensus can be the most likely outcome and the stock may still be attractive or unattractive. Companies with narrow outcomes require a non-consensus point of view for a buy or sell.


“Finance departments teach that volatility equals risk. Now they want to measure risk. And they don't know any other way-they don't know how to do it, basically. So they say that volatility measures risk.

I've often used the example of the Washington Post stock when we first bought it: In 1973, it had gone down almost 50%-from a valuation of the whole company of close to say $180 or $175 million down to maybe $80 million or $90 million. And because it happened very fast, the beta of the stock had actually increased. A professor would have told you that the stock of the company was more risky if you bought it for $80 million than if you bought it for $170 million-which is something that I’ve thought about ever since they told me that 25 years ago. And I still haven’t figured it out.”

~ Warren Buffett

Outstanding Investor Digest (August 8, 1997)

We love Warren Buffett. Anyone who’s ever read our research or heard us talk knows it to be true. But there is something that’s been bugging us for a long time, and we have to get it off our chests.

Buffett got this one wrong.

Often, when Buffett needs a lead-in to slam finance theory, he tells the above-quoted story as prima facie evidence of his case. In early 1970s, Washington Post stock got walloped, the beta went up (suggesting the stock was more risky) while any right-minded investor should see that the stock was actually less risky (because the price dropped more than the value).

Buffett’s argument has two problems. The first is that the beta didn’t go up: we have the empirical data to back that one. The second is that in saying the stock is less risky, Buffett assumed that the price to value gap had widened-the stock’s “margin of safety” grew. But value could be logically distinct from price only if Buffett believed something different than what the market believed. Buffett’s judgment proved to be correct, but that is not necessarily a statement about the shortcomings of finance theory.

Rest assured, Buffett faithful, this report will have a happy ending. Indeed, we believe all investors can learn a great deal about an appropriate investment philosophy by studying and practicing Buffett’s stock selection approach. But before we get to the good stuff, we have to address the issue of beta.

We are not enthusiastic defenders of the finance theory faith. In fact, we have argued that a new framework, based on complex adaptive systems, will supercede modern finance theory. But it’s one thing to attack finance theory based on what it predicts, it’s another game altogether to challenge the theory based on claims it doesn’t make. And nowhere does finance theory say that the beta on Washington Post’s stock must rise just because the stock declines. Such a statement confuses beta, a measure of a stock’s covariance vis-a-vis the market (often using the S&P 500 as a proxy), with alpha, a measure of risk-adjusted excess returns.

Exhibit 1 presents Washington Post’s beta and alpha graphically. Beta is the slope of the fitted line through the plotted monthly rates of return for Washington Post versus the S&P 500. Beta doesn’t measure an asset’s returns versus another asset, it just measures whether or not the asset’s price bounces more or less than another asset’s. Alpha, the intercept, does represent a rate of price change. So just because Washington Post’s alpha was negative (i.e., its returns were below those of the market) during 1973 didn’t mean that its beta had to rise.

Michael Mauboussin - Risk Beta Versus Margin Of Safety

Exhibit 2 shows the actual historical beta for WPO’s stock. Notwithstanding the stock’s correction, the beta actually dropped from roughly 2.4 in early 1973 to 2.3 by the end of the year, by our calculation. Barra’s beta figures show a similar decline. The negative alpha, of course, reflects the stock’s poor relative performance.

Michael Mauboussin - Risk Beta Versus Margin Of Safety

The classic definition of risk is the possibility of suffering harm or loss. So an asset that has a wide distribution of potential returns presents a greater probability of “suffering harm” (or sizable gains) than an asset with a narrow distribution of probabilities. We agree with Peter Bernstein, who notes that “volatility, or variance, has an intuitive

1, 2  - View Full Page