When you see an investor beat the market for 15 consecutive years, or maintain outperformance over several decades, what's your first thought? Lucky? Skilled? Just a random outcome?
Many investors, even professional fund managers, view beating the S&P 500 over the long term like coin flipping. They assume it's pure chance, that eventually everyone returns to average.
In "More Than You Know" (2007), Michael Mauboussin examines investment streaks and their probabilities. His analysis demonstrates that while short-term success might be random, sustained long-term outperformance signals genuine skill, not just luck.
While this may seem intuitive, many investors still make poor decisions based on flawed probability thinking:
- You might abandon a skilled manager after a bad year.
- You might dismiss your own decade of outperformance as temporary luck.
- Or you might trust a three-year hot streak that means nothing.
In this post, we'll discuss why your brain sees patterns where none exist and how investment success differs from random chance. We'll also analyze Bill Miller's famous 15-year streak and the flawed assumptions critics make about what these probabilities really mean.
Once you understand the real probabilities, you'll stop second-guessing your successes and start trusting the process that creates them.
Hot Hand Illusion
The human brain always searches for patterns, even in random sequences.
When a basketball player sinks several shots in a row, fans and players believe they have a “hot hand” that improves their chances of making the next shot.
This is called the “belief in the law of small numbers,” where people expect even short sequences to reflect overall probabilities.
Just like with basketball shots, show someone a coin flip with several heads in a row, and they'll start believing the entire sequence isn't random.
Why Investing Differs From Coin Tosses
Investment decisions involve vastly different skill levels that create predictable differences in success rates.
In a fair coin toss, each flip has exactly 50% odds. In investing, however, skill creates persistent advantages.
Consider Mauboussin's example of two basketball players:
- Sally Swish makes 60% of her shots. The probability of hitting five straight is (0.6)5 = 7.8%, occurring roughly every 13 attempts.
- Allen Airball makes only 30% of his shots. His chances of five straight drop to (0.3)5 = 0.24%, happening once every 412 attempts.
Without breaking any probability rules, Sally will have far more streaks than Allen.
The same mathematical concept applies to investing: skilled investors, like skilled basketball players, will naturally produce longer winning streaks than less skilled competitors.
Super Investors
We recently put together this visual that tracks 50 accomplished investors who beat the S&P 500 Total Return (SP500TR) Index over extended periods (most are 10+ years):Excess Returns and Investment Track Records
Although many of these investors don't have consecutive year streaks like Miller, their track records still support Mauboussin's core insight that sustained long-term outperformance (e.g., Warren Buffett, Mario Gabelli, Prem Watsa) signals genuine skill rather than temporary good fortune.
Bill Miller Debate
Misunderstanding the Odds
Bill Miller's Value Trust fund outperformed the S&P 500 for 15 consecutive years through 2005, achieving a 4.7% CAGR outperformance versus the SP500TR Index.
Critics dismissed this as luck. Notably, even bond manager Bill Gross (who co-founded PIMCO) claimed Miller's 12-year streak equaled rolling twelve sevens with dice, an event with actual odds of roughly 1 in 2.2 billion.
These critics apply the coin-toss metaphor incorrectly. They assume each fund has a 50% chance of beating the market each year. With only 900 comparable funds at the start of Miller's streak, they argue his success could happen through chance alone.
Mauboussin examined this argument by calculating probabilities under different scenarios:
The table shows their mathematical reasoning.
If 50% of funds beat the market annually, the odds of one fund succeeding for 15 years straight would be 1 in 32,768. Given 900 funds, you'd expect at least one to achieve this by pure luck.
Real Probabilities
The flaw lies in the 50/50 assumption. Most funds don't face even odds against the market.
Over Miller's 15-year period, only 44% of funds beat the market on average. Using this realistic percentage, the probability drops to roughly 1 in 223,000 (calculated as 1/0.4415).
But even this calculation understates the difficulty. Actual yearly data shows how difficult some years were for active managers:
As you can see, in 1995, only 12.6% of funds beat the market. In 1997, just 7.9% succeeded.
When Mauboussin calculated the cumulative probability using real percentages from each year, Miller's feat carried odds of about 1 in 2.3 million. This makes his achievement far more impressive than critics suggested.
Distinguishing Skill From Luck
As we've demonstrated with the coin-tossing metaphor, critics wrongly assume all fund managers have equal ability. But investing fundamentally differs from random chance because skill creates persistent advantages.
While luck plays a role in any success story, it cannot sustain performance over many years. The investors we track prove this with their multi-decade outperformance.
This distinction matters when evaluating fund managers and your own investment returns.
A three-year hot streak might mean little, as short-term movements often reflect market noise. But when someone beats the market for a decade or more, probability strongly suggests you're observing skill.
Moreover, as Mauboussin notes, maintaining consistent outperformance becomes increasingly difficult over time. The pressure mounts each year, and a single year of underperforming the S&P 500 could tarnish an otherwise impressive record.
So when investors like Bill Miller or those in our visual achieve decades of success, the probabilities confirm what common sense suggests: these aren't lucky coin flippers, but investors with genuine, sustainable talent.
Note: Smaller funds have an advantage because they can take meaningful positions in companies too small to impact larger portfolios. Different market environments also favor different investment styles. Still, the core principle holds: sustained multi-decade outperformance signals skill regardless of these variables.
