Careful With That Extrapolation!

Consider these four quotations:

“In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”
- Mark Twain in Life on the Mississippi

“If a bacteria cell divides every twenty minutes, then in 24 hours a single bacteria cell will produce a colony of bacteria which weighs more than planet Earth.” (Grade 6 Science textbook)

“At this rate of adoption, every man, woman, and child on the planet will have two cellphones each by 2014 – and an iPad.” (Industry trade magazine)

“The lily pad doubled in size every day.  It completely covered the pond on day 23, but hardly anyone noticed it until day 20 when it covered  1/8^th of the pond.  By then it was too late to do anything.”

In the real world of growth and declines, we have to pay attention to trends and possible extrapolation in order to decide how to act.  Extrapolation is a necessary skill for managers in order to assess situations and proactively plan for the future.

But the Mississippi River is still over 2300 miles long, more than a hundred years after Mark Twain penned Life on the Mississippi.  Bacteria colonies have never weighed as much the planet.  There are many reasons why cell phones will not be uniformly distributed worldwide.  Lily pads will not grow forever. 

Here’s the key: Pay attention to constraints and counter-balancing trends as you consider extrapolations on trends.  Think about limiting factors to growth (or decline).  Don’t make linear or exponential extrapolations assuming conditions for that growth or decline remain constant.