Extrapolation

Whenever I heard predictions about the far future, I'm reminded of the following clip from The Simpsons. If I was really on the ball, I'd find the Youtube clip, but I've got a limited amount of time, so y'all need to settle for the transcript:

Meanwhile, at Disco Stu's "Can't Stop the Learnin' Disco Academies"...

Disco Stu: [making indescribable body motions]  Did you know that disco
          record sales were up 400/ for the year ending 1976?  If these
          trends continue...  A-y-y-y!
           [kicks his feet up on his desk wearing see-through platforms
            with water and fish inside]
   Homer: Uh, your fish are dead.
Disco Stu: Yeah, I know.  I... can't get them out of there.

http://www.snpp.com/episodes/4F08.html

There are two important things to learn. First, extrapolation, even if tends continue, results in extremely poor predictions. Errors will compound, and changes in functional form will break down models. Second, extrapolation cannot account for changes which completely break trends. Wang Laboratories had annual revenues in the billions, but failed to foresee the rise of the personal computer.