Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Funny thing I've found in David Griffiths QM textbook

  1. Jun 21, 2015 #1
    I really like the book, its the first physics textbook that I liked actually. But I've found a minor error.
    On page 8 (chapter 1 The Wave Function) it says that if you sum deviations from average of a random variable you'd get zero because " Δj is as often negative as positive", here's the formula:
    [tex] \Delta j=j-<j> [/tex]

    But I think that's not the reason you will get zero, it need not deviate the same number of times in both direction from the average, but in that case number of deviations on one side will be canceled by the magnitude of deviations on the opposite side, I think Griffith mixed median with an average.

    Last edited: Jun 21, 2015
  2. jcsd
  3. Jun 21, 2015 #2


    User Avatar
    Science Advisor

    You are right.
  4. Jun 21, 2015 #3
    The reasoning Griffiths gives is indeed incorrect, but the result is still true: summing deviations from the average gives you zero.
  5. Jun 21, 2015 #4


    User Avatar
    Science Advisor
    Gold Member

    In a sense, Carlin is right, though I doubt he was aware of this when he made his comment: IQ ( a measure of intelligence/stupidity) is normally-distributed, so that the mean/average and median coincide.
  6. Jun 22, 2015 #5
    On the idealized curve yes, on the actual data no. Besides it's a joke.
  7. Jun 22, 2015 #6


    User Avatar
    Science Advisor
    Gold Member

    Ah, sorry, I did not get it the 1st time. I guess a Carlin quote should have made it clear.
  8. Jun 22, 2015 #7
    http://cdn.alltheragefaces.com/img/faces/large/happy-yes-l.png [Broken]
    Last edited by a moderator: May 7, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook