Correlation is a tricky subject. It is the measure of the mathematical
relationship between two measurements.
These could be any measurements, and it becomes tricky when someone
tries to attribute a physical relationship to this mathematical relationship,
that is, they try to tell you that because two things change at the same time
(increase or decrease) that one causes the other.
A common example is carbon dioxide (CO2) in the atmosphere
and climate. Over many thousands of
years, when there was more CO2 in the atmosphere, global temperature has been
higher. It’s not possible to deduce
immediately from that the one causes the other.
Scientists must find the mechanism by which the change in one leads to a
change in the other – the greenhouse effect of reflecting the earth’s heat back
toward the ground, for example. But
correlation alone is not good enough. It
only provides a clue, a reason for further investigation.
Unfortunately, we read about correlation, these
relationships, frequently but the reporters are not careful to clarify this
important point. A recent example comes
from the University of Texas. “Fathers
who worked in engineering were two times as likely to have a child with an
autism spectrum disorder (ASD). Those who worked in finance were four times
more likely and those who worked in health care occupations were six times more
likely to have a child on the autism spectrum.”
This is correlation, a mathematical relationship. Have they looked for a mechanism to explain
their results? Is it an inheritance
issue? Is it the way the father treats
his children? Is there any real-life
relationship at all? The article says
nothing about it. Should someone opt not to become a doctor for fear of
increasing his children’s odds of being autistic? Does one cause the other; do they have a
common cause; or are the findings of the study based on an odd coincidence?
Stranger coincidences are really quite common. This link shows a few examples from “a law
student at Harvard who, in his spare time, put together a website that
finds very, very high correlations between things that are absolutely not
related, like margarine consumption and the divorce rate in Maine.” My favorite is a 96.97% correlation between deaths by getting tangled in bed sheets and the
revenue of ski facilities. This is
extremely close to perfect correlation of 100%, and much better than scientists
that present studies for publications usually achieve.
Readers of any scientific or medical reports beware. A correlation is only the beginning of an investigation, not a conclusion.
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