Today, our meters fool us. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.
In an older episode about
analog and digital
watches, I made the offhand remark that digital
watches report the time with greater precision.
When a listener wrote to point out that a digital
watch can be just as wrong as an analog watch, I
realized that I'd used the word precise in its
technical engineering sense.
We use the two terms accurate and
precise to mean quite different things.
Imagine for example, two police officers on the
target range. When they look at their respective
targets, the first discovers a very tight grouping
of hits, four inches to the left of the center. The
second officer finds a random scatter of hits,
eight inches in diameter, centered about the aiming
point.
And so the first one's shooting is extremely
precise, but it's inaccurate. The second one is
quite accurate and very imprecise. The word precise
actually means no more than well-defined.
We save the word accurate for
close-to-being-correct.
This issue took on special importance back in the
mid 1970s when we all gave up our slide rules in
favor of pocket calculators. Slide rules were
precise within only a percent or so. Now, in a
blink, we could divide two numbers and get a
ten-decimal-place result. The result was both
precise and accurate.
Or was it accurate? Our work almost always depends
on measured data -- like conductivities or
yield-stresses -- which are usually a few percent
off. Students were now reporting calculations to a
precision of a ten thousandth of a
percent, when they could not possibly be
accurate within one percent.
This sort of thing doesn't stop in the engineering
office. I always grow nervous when someone praises
a person's precision of language, for the same
problem applies. Back in 1950, Senator Joe McCarthy
made his famous speech in Wheeling, West Virginia.
He announced that 205 State Department employees
were communists who reported directly back to the
Kremlin. His statement was wonderfully precise, and
wholly inaccurate.
On the other hand, when poet John William Burgon
called the ancient city of Petra,
A rose-red city half as old as time,
he spoke without any precision, but with
a curious kind of accuracy.
This issue dogs us all as we read political
statements, advertising copy, or any attempt to win
us over to various causes. We are constantly
dazzled by precision, while accuracy is absent. And
I'm back to digital readouts.
Film critic Roger Ebert likes to talk about the
hackneyed scene where clock with a red LED ticks
down toward zero. That's when, say, scientists have
predicted that a chemical reaction will complete
itself and blow up the universe. The despairing
hero looks at the reactor's wiring and says,
Shall I cut the green wire or the red?
Wouldn't it be something to see, just once, a hero
who shrugs and says, Wait a minute; the
prediction's only approximate. I can take another
minute and get it right.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
These issues are treated very early in any
engineering measurements textbook. See, e.g., T. G.
Beckwith, R. D. Marangoni, and J. H. Lienhard, V,
Mechanical Measurements, 5th ed.. Reading,
MA: Addison-Wesley Publishing Co., 1993.
