When Rare Events Aren’t All That Rare

 | Jan 21, 2013 06:30AM ET

It’s Myth-Busting Monday. And that means it’s time to tackle a topic that’s potentially contentious.

Today, I’m taking on the idea of rarity in the financial markets. Specifically, whether or not “rare events” truly end up being a rare occurrence.

While the findings might not surprise you, the implications for our investing strategies certainly will. So let’s get to it.

“Unlikely” Doesn’t Mean “Never”

By definition, rare events should seldom occur.

Applying that understanding to financial markets, however, assumes that all market events follow a normal distribution. Or, in layman’s terms, a bell-shaped curve. (If you’re still clueless, fetch your Statistics 101 textbook from the attic to achieve enlightenment.)

More specifically, the statistics say that 99.7% of all daily movements should fall within three standard deviations of the mean, no more.

Well, guess what? New research suggests that they clearly don’t follow such a pattern.

Deutsche Bank recently measured the occurrence of rare events – defined as daily movements of three standard deviations or more from the mean – for various markets. Like stocks, 10-year Treasuries, the U.S. dollar/Japanese yen exchange rate… the list goes on.