That basic knowledge, along with some understanding of accounting and a little bit of study, would enable one to evaluate and invest in any number of restaurants and restaurant chains, public or private. It’s not all that complicated.

1996 Shareholder Letter:

What an investor needs is the ability to correctly evaluate selected businesses. Note that word “selected”: You don’t have to be an expert on every company, or even many. You only have to be able to evaluate companies within your circle of competence. The size of that circle is not very important; knowing its boundaries,

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Our circle of competence can be widened, but only slowly and over time. Mistakes are most often made when straying from this discipline.

You have to figure out what your own aptitudes are. If you play games where other people have the aptitudes and you don’t, you’re going to lose. And that’s as close to certain as any prediction that you can make. You have to figure out where you’ve got an edge. And you’ve got to play within your own circle of competence.

It did not hurt Mrs. B to have such a narrow area of competence. In fact, one could argue the opposite. Her rigid devotion to that area allowed her to focus. Only with that focus could she have overcome her handicaps to achieve such extreme success.