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A columnist of The New York Times wondered why tax
incentives were going to be given to buyers of an “hypothetical
hybrid Dodge Durango that gets 14 miles per gallon instead of 12,
thanks to its second, electric power source” but not to a “buyer
of a conventional, gasoline-powered Honda Civic that gets 40 miles
per gallon.” (J.L. Kitman, NYT, April 16, 2006).
Isn’t the Civic more
fuel-efficient than the Durango hybrid? Wrong.
In the June 20, 2008 issue of
Science, Richard P. Larrick and Jack B. Soll of Duke University in
Durham, North Carolina argued that a 2-mile per gallon (MPG)
improvement for the Durango results in less gas consumed than the
improvement of a 28-MPG Civic to a 40-MPG version.
Using 10,000 miles as the common
distance for all 4 vehicles, the 12-MPG car will consume about 833
gallons (10,000-12) while the 14 MPG hybrid will use 714 gallons
(10,000-14) or roughly a difference of 120 gallons.
Over the same distance, the
28-MPG vehicle will use 357 gallons while the 40-MPG model will burn
250 gallons—a difference of 107 gallons.
Dodge wins over Honda by 13
gallons.
Larrick and Soll made 3 points.
The first is the false belief by
most people that the amount of gas consumed by a car “decreases as
a linear function of [its] MPG.” The relationship is not linear
but curvilinear.
Their second point is eliminating
the most fuel-inefficient vehicles or making even small improvements
on them should be the object of policy and not fuel efficiency in
terms of gas used over a given distance.
But it’s the third point
that’s the eye-opener—the prevalence of linear thinking among
well-informed and presumably analytical people.
To probe it, Larrick and Soll
conducted 3 experiments.
In the first, 77 randomly
selected college students were asked “to assume that a person
drives 10,000 miles per year and is contemplating changing from a
current vehicle to a new one.” Their task was to rank 5 pairs of
old and new cars according to their judgment of which new car
“would reduce gas consumption the most compared to the original
car.”
The result was a majority
confirmation of the linear relationship to MPG rather than the
actual reduction in gas consumption.
The second experiment was to test
whether the price that people would pay for more efficient cars
would also show a linear relationship to MPG. A random selection of
students were asked to choose from several vehicles that were
identical except for their fuel efficiency. The assumption was still
10,000 miles per year for distance; a baseline model that costs
$20,000 gets 15 MPG. The participants were told to state the highest
price that they would be willing to pay for each of 5 vehicles that
differed only in MPG. Again the linear relationship to MPG won by a
majority over actual reduction in gas consumption. Cars that were
improved to get 19 to 25 MPG were “undervalued” while those with
improvements to 55 MPG were “overvalued.”
For the last test, the metric was
changed from MPG to gallons per 100 miles (GPM). The survey was done
on-line with 171 participants “drawn from a national subject
pool.” Their ages ranged from 18 to 75, with 35 as the median.
They were given the following scenario.
“A town maintains a fleet of
vehicles for town employee use. It has two types of vehicles. Type A
gets 15 miles per gallon. Type B gets 34 miles per gallon. The town
has 100 Type A vehicles and 100 Type B vehicles. Each car in the
fleet is driven 10,000 miles per year.”
The participants were asked to
choose a plan for replacing the original vehicles with hybrid models
with the “overriding” aim of reducing gas consumption in order
to reduce harmful emissions.
One group of 78 randomly assigned
participants was asked to make a recommendation based solely on MPG.
They had 2 options. Option 1, replace the 100 vehicles that get 15
MPG with vehicles that get 19 MPG. Option 2, replace the 100
vehicles that get 34 MPG with vehicles that get 44 MPG. Quick
arithmetic would show that fuel efficiency is better in Option 1 by
14,035 gallons than in Option 2 by 6,684. But as in the 2 other
experiments, the majority—75 percent—chose Option 2.
Participants in the GPM group,
also randomly assigned, were given the same instructions with the
addition that miles-per-gallon had been changed to
gallons-per-100-miles.
Type A vehicles use 6.67 GPM.
Type B vehicles consume 2.94 GPM. To state this for hybrid vehicles
the choices were (Option 1) replace 200 vehicles that get 6.64 GPM
with vehicles that get 5.26 GPM and (Option 2) replace 100 vehicles
that get 2.94 GPM with vehicles that get 2.27 GPM. The majority, 64
percent, chose Option 1, which offers a small gain in MPG but better
fuel efficiency. Overall, the percentage that chose the more
fuel-efficient option rose from 25 percent in the MPG group to 64
percent in the GPM group.
These experiments showed that
most people misunderstand the MPG as a measure of fuel efficiency.
Linear reasoning consistently undervalued small improvements on
inefficient vehicles.
The authors conclude that for
policy it’s preferable “to express fuel efficiency as a ratio of
volume of consumption to a unit of distance.”
Larrick and Soll are right. MPG
should not be the basis of policy on fuel efficiency nor a criterion
for calculating reductions of greenhouse gas emissions. But as
important, or perhaps more important, are habits of thought that go
beyond decisions on which vehicle to buy. Linear thinking and
reasoning are all too common among all social classes resulting in
unexamined premises and wrong conclusions on sensitive issues.
opinion@manilatimes.net
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