The tyranny of the single-number index

TreeHugger’s Collin Dunn offers up this simple comparison of the costs of transportation in a short, short post:

52.2 — the average cost in the US, in cents per mile, of driving a car alone, according to AAA. Compare that to…

20.7 — the average cost in the US, in cents per mile, of riding public transportation, according to the American Public Transportation Association.

Which reminded me of this column from last weekend, by the Globe and Mail‘s Neil Reynolds (full disclosure: for whom I once briefly worked), on a study compiling the total energy costs of many, many cars, from conception to being driven. The results from CNW Research were expressed in easy-to-grasp dollar figures:

Compare the SUVs against the hybrids and you get a sweep in favour of conventional technology. The best-rated smaller SUVs are more than twice as eco-friendly as the hybrids: Dodge’s Durango, $1.57; Ford’s Explorer, $1.61; Chevrolet’s TrailBlazer, $1.61; Jeep’s Grand Cherokee, $1.80.

More remarkably, one of the larger SUVs, Ford’s Expedition, beats the hybrids with an eco-cost of $3.54.

CNW found wide differences, however, within classes of vehicles. For 18 models of luxury cars, the average energy cost is $4.45. Yet the best of these luxury cars are superior, in lifetime energy use, to hybrids.

Both sets of figures come loaded with caveats.

CNW Research even included a note (PDF) saying its figures for cars such as the Prius could be misleading, since so much original work had been done on the Prius that has only been spread out over a couple of years and one model, whereas Hummers, for instance, use a lot of established technology (adding less energy in the design phase) and spread the costs of innovation across several models.

At TreeHugger, the comments thread is full of “But what about..?”s and “This doesn’t consider…”, most of which complaints are true. The cost of taking public transportation is heavily and directly subsidized, for instance, and that’s not reflected in the price. But then, so’s the cost of driving, on smooth and well-signaled roads, and that’s not reflected in the price, either. People pay for both out of their taxes, so they’re real costs, just not coming out of people’s wallets right off. Besides, who cares what the average cost is? What’s it cost me? is the real question.

But nobody’s going to remember these warnings and reservations. Instead, we have masses of complex information condensed into single, easily remembered, inherently problematic and frequently misleading figure.

Consumers need information like this, but to make useful decisions, simple one-number indices aren’t good enough.

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