Buffett's rule, from his 1996 letter to Berkshire shareholders, sounds plainer than it is. You don't have to be an expert on every company, or even on many. You need only "be able to evaluate companies within your circle of competence." The size of that circle matters little. "Knowing its boundaries, however, is vital." Your circle of competence is the set of businesses you can really understand. You can explain how the company makes money and what drives it, as well as what could break it. Above all, you know what would make you change your mind. The circle can be modest. You just have to know where its edge is and pass on everything outside it without embarrassment.
Why do mistakes happen at the edge of the circle?
Why does the perimeter matter more than the area? Errors don't happen deep inside the circle, where you understand things well. Nor do they happen far outside it, where you know to stay away. They happen at the boundary, in the seductive zone where you understand a business just well enough to form a thesis but not well enough to see what you're missing. A hammer-wielder lives there permanently, and even good analysts get hurt there. Picture the software investor who "gets" a fintech because it has an app, and misses that it's really a balance-sheet lender exposed to a credit cycle he has never studied. Or the consumer investor who buys a specialty pharma "because everyone needs medicine" and gets destroyed by an FDA import alert whose significance he couldn't read. The boundary is dangerous because it doesn't feel like a boundary. It feels like competence. So the antidote is to make the perimeter explicit and written down. Then "am I actually inside my circle here?" becomes a checklist question instead of a feeling.
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How do you write your circle of competence down?
Writing it down forces three kinds of honesty. First, what's really inside: sectors and business models where you've done the work, can spread the financials by hand, know the KPIs and their thresholds from Phase 5, and have studied the failure cases. Second, what's at the edge. These are things you're actively learning and can hold only with an extra margin of safety and a smaller size. Third, what's outside. That list should be long and unashamed. Buffett famously kept technology outside his circle for most of his career and let Microsoft, Google and Amazon go by instead of pretending. The same restraint that "cost" him those kept him out of the entire dot-com wreck. Here's an Indian version. An investor who has never mastered asset-liability management should keep leveraged NBFCs and housing-finance companies firmly outside the circle. The IL&FS/DHFL complex looked like a set of cheap, growing lenders right up until the funding side gave way, and the funding side was the whole game. Call it timid if you like. Staying out of what you can't evaluate is the single cheapest source of avoided losses in investing.
Can your circle of competence grow?
The circle can grow. Growing it, sector by sector, is what the whole course has been doing on purpose. But it grows slowly, through real work on real filings. Reading a headline won't do it. There's a subtle trap, too. Knowing the business well isn't the same as knowing the stock. You can understand a company perfectly and still have no edge if everything you know is already in the price. The variant view makes that sharper. Your written circle-of-competence statement is the first deliverable of the mini-project and a permanent input to your personal checklist. Draft it honestly. A two-line circle you actually respect beats a two-page one you routinely ignore.