📖Charlie Munger

Regression to the Mean

🌿 Intermediate★★★★☆

Extreme performance — good or bad — naturally reverts toward the average over time.

💬

Regression to the mean is the most powerful law in statistics.

— Munger on Statistics，2005

🏠 Everyday Analogy

Just like flipping a coin, after landing on heads ten times in a row, the probability of the next flip being tails remains 50%. However, over the long run, the outcomes will tend to balance out. The stock market operates similarly: stocks that have surged continuously will eventually pull back, while those that have declined persistently will rebound. This is not a matter of luck but a statistical regularity.

📖 Core Interpretation

Extreme performance tends to revert to the mean, which is a fundamental principle of statistics.
💎 Key Insight：A fund manager with three amazing years is likely reverting to luck, not skill. A company with terrible results may be bottoming out. Regression to the mean is one of the most reliable statistical patterns, yet most investors ignore it. They chase hot funds (which cool off) and avoid cold funds (which warm up). Understanding this tendency prevents costly trend-chasing.

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❓ Why It Matters

Helps distinguish genuine skill from luck, preventing overreaction to short-term performance.

🎯 How to Practice

Be skeptical of anomalous performance; long-term trends hold more reference value than short-term results.

🎙️ Master's Voice

We both insist on a lot of time being available almost every day to just sit and think.
Munger and Buffett spend most of their time thinking, not doing. Quiet reflection produces better decisions than constant activity.

⚔️ Practical Guide

✅ Decision Checklist

  • Do I have time to think?
  • Am I too busy to reflect?
  • Is my schedule conducive to thinking?

📋 Action Steps

  1. Schedule thinking time daily
  2. Reduce meetings and noise
  3. Value reflection over activity

🚨 Warning Signs

  • No time for thinking
  • Constant busyness
  • Action over reflection

⚠️ Common Pitfalls

✕Not all things revert to the mean.
✕Structural changes may alter the mean.

📚 Case Studies

1
Dot-com Highflyers Revert (1999)
Momentum tech stocks with no earnings soared in late 1990s, far above intrinsic value. Munger cited how extreme success often attracts excessive capital and optimism.
✨ Outcome：Most collapsed 2000–2002, reverting toward business fundamentals; diversified investors later buying quality at reasonable prices earned better long-run returns.
2
Housing and Financial Stocks (2007)
US homebuilders and mortgage-related financials enjoyed record profits and valuations pre-2008, driven by easy credit and belief that housing prices couldn’t fall nationally.
✨ Outcome：Mean reversion hit: defaults surged, profits evaporated, and prices crashed 2007–2009. Investors who assumed abnormal profitability would persist suffered large permanent losses.

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