Recently, many people have been discussing position sizing issues in investing. In fact, many mistakes made by retail investors are the same—only thinking about "rising" or "falling," completely ignoring money management. Why do top professional gamblers and investors never go all-in? The answer lies in the Kelly formula.

Let me start with a core concept: investing is fundamentally probability theory, not gambling. What's the difference? In gambling, you might lose everything, but in investing, losses usually only mean losing part of your capital (unless the asset drops to zero). This difference is crucial and is also why the Kelly formula can be useful.

The Kelly formula tells us three important things. First, don't be greedy. Even if your win rate is high, going all-in on one shot can wipe out your capital with just one failure. Second, don't be overly conservative. If the opportunity is truly good, increasing your position appropriately can earn more. Third, adjust dynamically. When you have more capital, you can bet more; when you have less, bet less—that's the secret to long-term survival.

The formula itself isn't complicated: f = p/l - q/g. Where f is the optimal proportion of your capital to invest, p is the probability of an increase, q is the probability of a decrease (q=1-p), g is the magnitude of the increase, and l is the magnitude of the decrease. Plug these numbers in, and you can calculate how much to bet each time.

When applying it practically, watch out for a few pitfalls. First, the accuracy of the Kelly formula entirely depends on how well you predict probabilities and odds. Overestimating your win rate or payout ratio can lead to traps. Second, if the suggested position exceeds 100%, it indicates a trap—there's no such thing as a guaranteed no-loss opportunity in reality. Many experienced traders use a "half-Kelly" strategy—betting only half of the suggested proportion—to further reduce risk.

When someone tells you to go all-in, remember to use a calculator and run the Kelly calculation. You'll find that true investing experts never rely on luck but on scientific capital allocation. The Kelly formula is like a smart fund manager, helping you make the optimal decision with each bet, maximizing long-term gains while avoiding bankruptcy.

In short, the essence of the Kelly formula is: bet heavily on good opportunities, but always keep enough capital to withstand bad luck.