In 2009, we had a big debate about whether to pass a stimulus package. Many esteemed and/or Nobel Prize-winning economists like Joseph Stiglitz, Larry Summers and Christina Romer argued that it would help lift the economy out of recession. Many other esteemed and/or Nobel Prize-winning economists like Robert Barro, Edward Prescott and James Buchanan argued that positive effects would be small and the package wouldn't be worth the long-term cost.

We went ahead and spent the roughly $800 billion. What have we learned?

For certain, nothing. The economists who supported the stimulus now argue the economy would have been worse off without it. Those who opposed it argue that the results have been meager. It's hard to think of anybody whose mind has been changed by what happened.

This is not entirely surprising. Nearly 80 years later, it's hard to know if the New Deal did much to end the Great Depression. Still, it would be nice if we could learn from experience. To avoid national catastrophe, we're going to have to figure out how to control health care costs, improve schools and do other things.

Jim Manzi has spent his career helping businesses learn from experience - first at AT&T Laboratories, then as a consultant with Strategic Planning Associates and then as founder of Applied Predictive Technologies, a successful software firm.
In his new book, "Uncontrolled," Manzi notes that many experts tackle policy problems by creating big pattern-finding models and then running simulations to see how proposals will work. That's essentially what the proponents and opponents of the stimulus package did.

The problem is that no model can capture enough of the world's complexity to yield definitive conclusions or make nonobvious predictions. A lot depends on what assumptions you build into them.

David Fuller's view At the risk of sounding glib, the world is more complex than ever before but at least we have the markets. Invest in companies that are increasing profits and dividends, and which show relative strength.