Tuesday, May 26, 2009

Meaningful Numbers on Commuter-Airline Safety

From The New York Times, in Pilots’ Lives Defy Glamorous Stereotype, we learn:

[O]f the six scheduled passenger flights that have crashed since Sept. 11, 2001, only one has been from a major carrier. Four, including the one in Buffalo, were commuter flights; a total of 133 people died on those flights. (The fifth, a 50-year-old seaplane in Miami, was in neither category.)

That sounds bad. However, missing from the story is the context necessary to understand the numbers. For example, what if commuter airlines flew four times as many flights as major airlines? Then the 4 to 1 ratio of commuter-airline to major-airline crashes would be expected. (We will ignore the fact that the tiny number of cases makes extrapolating fuzzy at best, and instead be thankful that the data on this topic is sparse.)

To the rescue we have Ben Sherwood’s Wing and a Prayer: How Safe is My Next Regional Plane Flight?, from The Huffington Post. According to Sherwood, commuter airlines fly roughly the same amount of flights as the major airlines. But wait, Sherwood goes the extra step to find what appears to be the more relevant numbers:

For the absolute latest on the risk of death on a regional carrier [Sherwood’s term for commuter airline], I checked with Arnold Barnett, a brilliant MIT professor who happens to be afraid of flying and who specializes in statistics on aviation safety.

Barnett points out that all the news this week about the Continental crash and safety questions about regional carriers have blurred an important distinction between jet and propeller aircraft.

“Historically, the safety record for piston and prop-jet aircraft has not been as good as that for pure jets,” Barnett says. “US regional jet flights have a splendid safety record,” he goes on. “They have suffered only one fatal crash in the past two decades.”

According to Barnett’s analysis, your risk of death on your next regional jet flight in the US is 1 in 30 million. In other words, you can travel every day for the next 82,191 years—on average—before you will die on a regional jet. (For comparison, your chance of dying on your next trip on a major carrier—one of the big airlines—is 1 in 60 million).

Prop-jets—planes with propellers driven by turbo-jet engines—are a different story, Barnett points out. Your risk of death on your next prop-jet flight, he says, is 1 in five million. Yes, the risk is greater than a jet flight, but you can still fly every day for a very very very long time before you run into a problem.

Thank you for the perspective, Arnold Barnett and Ben Sherwood.

Sunday, May 17, 2009

Spread the Word: Affordance

From one of of our bathrooms at home, the shower/tub controls are pictured at left.

The handle pointing to five o’clock regulates the water flow. The knob pointing to one o’clock sets the temperature. But how do you activate the shower?

Look at the picture again. What do you think?

The answer is, you pull down on the tub spout’s ring, where the water comes out. This plugs the tub spout and directs the water up to the shower head.

Functionally it makes sense, but the only thing obvious about this feature is its need for a better affordance: a design element that shows the user what to do.

The computer-user-interface expert Don Norman popularized the term affordance in his book The Design of Everyday Things. One of his classic examples was the door that you pull, only to find that you need to push it. Why did you pull? Probably because it had a pull-style handle, when it should have had a flat plate. The latter can only be pushed. (A rule of thumb: If a door needs a sign to tell you “push” or “pull,” it could use a better affordance.)

Although the term sounds jargony, affordance provides a shorthand for thinking and talking about what would otherwise be referred to as, “something that shows you what to do with it.” The term is used primarily by certain types of designers. We would all benefit if a wider array of professionals understood and internalized the term. Spread the word.