Today, let's figure out how to plot a graph. 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.
Yale professor Edward Tufte
has long argued that to be understood, data must be
seen -- not just read. Facts alone tell us little.
Facts need context. We need to build the right
display case to explain (or even to understand) our
Example: Suppose the Dow-Jones Index has risen
slowly over a month's time. Then, on Thursday, it
falls 80 points. Nothing verydramatic there. Eighty
points out of 8000 is only one percent. But the
next day the newspaper plots the month's history.
To make changes clear, they use a range from 7800
to 8200 points.
Now that one-percent drop looks like the Grand
Canyon, and naive readers panic. A faulty display
of perfectly valid data has led people far away
from what the data really say.
So Tufte turns to Richard Feynman dipping an O-ring
into ice water at the senate hearings on the
Challenger disaster. Testimony about effects of
temperature on O-ring material had been droning on.
It was no secret that an O-ring had lost elasticity
needed to seal a joint. Yet the fact had not really
been made clear.
Then Feynman clamped an O-ring and forced it into a
glass of ice water so it was bent. When he withdrew
it, it didn't spring back. Cold had robbed it of
its function. Suddenly laymen understood in their
guts what data had failed to tell their heads.
Still, that demonstration, like the Dow-Jones
graph, was flawed. Remember, data need context.
What is normal behavior for an O-ring? We should
force a second O-ring into a glass of
room-temperature water. We need to know normality
to understand abnormality. Feynman succeeded, but
it wasn't rigorous science.
Data need contexts. Years ago a friend who wrote
music-history texts looked at me sadly and said,
You know, John, we do it wrong in music schools.
We expose our students to Bach, Josquin, Mozart.
How can they develop their powers of discrimination
without hearing bad music as well?
That always struck me as being
powerfully insightful. Information removed from any
useful backdrop is meaningless.
Tufte returns to the Challenger failure. Why didn't
engineers and managers scrub the launch? Not only
were O-ring properties well known, but so was a
previous history of trouble. So he plots a graph.
He shows the extent of O-ring damage on previous
launches as a function of ambient temperature at
launch time. The curve rises stomach-wrenchingly as
the temperature falls. And on January 27, 1986, the
temperature was far lower than it had ever been.
But he plotted that graph in hindsight. It wasn't
there at the time. Tufte finishes with guidelines
for visual displays. Keep cause and effect in mind,
document your sources, and so forth. But one last
bit of advice lurks behind the rest. It is: see
what others cannot see. And that is never easy
advice to follow.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds