80% certainty (or better)

Compound interest is a process that you can be fairly confident in. Given a long-enough time frame (and the wherewithal to let it sit and wait), you can be confident you’ll get a return on your investment in the distant-but not too distant-future.

And for those looking to get fit, dieting (correctly) and working out (consistently) follow a similar trajectory. It might take you longer than others, but you can be fairly certain that doing these things will give you results.

Compare this to time spent gambling in a casino. The time you’re there (and the amount you spend) isn't at all correlated with how much you'll have when you exit the building. There's a high margin of error with your odds of success being constantly reset for every game you play.

Or consider a liberal arts degree. Sure, it might help you become the next David Foster Wallace, but can you be at least 80% sure it will land you job that justifies the cost of tuition? Not likely.

There comes a time where we all have to take risks (financial, emotional, or otherwise) to get what we want in life. Which means that, going along with hedging one’s bets, it makes a lot of sense to try to invest in pursuits where you can be sufficiently confident in the process. I.e., 80% certain or better.