It's actually quite difficult to identify above average.

ISTR a study a few years ago that said the only way you can know for certain that a fund manager is above average is on something like 30+ years of data. There's too much random variation.

I think it was the same article that said the baseball season would need to be several hundred games before you could definitively choose the best team in a given season.

Stepping back even further, a lot happens over 30 years. Wars, social change, major political movements, tectonic shifts in technology, etc.

Perfect example: Buffett has always touted value as a great investment strategy, but he completely missed the cocktail of global trade, speed, and scale that have driven Amazon, Google, Uber, Netflix, Apple, etc. to grow revenue and profits so rapidly.

Looking back, I think this shift into a world where it's possible for a company to go from zero to hundreds of millions/yr in net revenue over a span of 4-5 years (Uber) is really going to be seen as some kind of historical turning point in business, on par with the invention of the printing press, or electricity. Companies used to take a century to accomplish growth that's now possible in less than a decade.

Which is all to say, over a long enough timespan, you're talking about a pretty deep and complex philosophical question, what it means to be a "good investor" in the face of so much change, risk, etc. To be honest, I'm not sure it's something that's knowable at all.

I think it's actually supposed to be quite difficult to see who is above average. At least in "A Random Walk Down Wall Street", they cite some statistics about how only 20% (?) of managers beat the market over a period of time, but it isn't the same people who do it over different periods of time.