Monday, August 29, 2011

Is it the end of the world?

The last few weeks have seen some usual events befall the Washington DC area; notably, the earthquake (where in the heck did THAT come from??!?) and Hurricane Irene.

Irene was potentially a biggie, but in terms of damage, it didn't bring nearly the water that Hurricane Isabel did in 2003.  I remember that vividly because I was living on a boat at the time in Southwest DC at the Gangplank Marina.  To this day, I'm still upset that I didn't document that event better -- there's nothing quite as surreal as waking up one morning to see someone riding their dinghy across Hains Point.  It was completely submerged.

But anyways, we've had a rash of seemingly nasty weather come DC's way recently.  Is this a sign of something taking a turn for the worse?  Are we all doomed?

I decided to run some numbers.  Let's take seven natural disasters:

Of course, some areas are more prone to these than others.  What are the chances that, in some small period of time, you get at least one of these at the hundred-year level?  As in, "Hurricane Isabel was the worst flooding in DC in 100 years."  Or, "the earthquake in August in DC was the worst in 150 years."

For each of these seven events, there is a 0.99 chance that it won't happen this year (for the once-in-a-hundred-years event).  So if you multiply the 0.99 for all seven events, there is a 93% chance that none of them will happen at the 100-year level.  Then, if you hang around DC for ten years, that's

0.93 x 0.93 x 0.93 x 0.93 x 0.93 x 0.93 x 0.93 x 0.93 x 0.93 x 0.93

= 0.495

That means, chances are that if you live in DC (or anywhere, for that matter) for 10 years, there is a better than 50% chance you will experience a once-in-a-hundred-year event of one of those seven catastrophes.

For the statistically minded and rigorous folks out there, I admit this is somewhat contrived.  I arbitrarily picked 7 events.  If you pick 12 events (maybe throw ice storms, hail storms, heat waves, continuous days of rain, and locust swarms in there), you'll get to the 50% level in 6 years.  And since the news media is always looking for the headline, "WORST [SOMETHING] IN 100 YEARS," you could say that they're casting their net pretty wide and looking for anything.  If your event database is 70 items large, you're likely to get a 1-in-100 year event every year.

So, no, I don't think the world is coming to an end.  We're just better at looking for it.