A reader asked the other day, "How much time do you need before you can separate skill versus luck in investing?"
My answer was "probably 20-30 years," which he found astounding. He thought I'd say five years. But here's my reasoning.
If a doctor performed one successful surgery, you can be pretty sure he's an expert. If he does one successful surgery every day for a year, he clearly knows what he's doing.
Investing is different. There are thousands of stocks, and at any given time, a fair number of them will be exploding higher. With millions of investors, some will be holding disproportionate amounts of those winners at any given moment. It can take five or 10 years of successful returns for an investor to make a case that results aren't entirely due to chance.
But even then -- with, say a 10-year track record of success -- an investor can't claim expertise. Or at least reliable expertise you'd expect from a doctor or an engineer. That's because the world is always changing, and th
