# Funny thing I've found in David Griffiths QM textbook

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1. Jun 21, 2015

### Alexandre

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:
$$\Delta j=j-<j>$$

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. Jun 21, 2015

### mathman

You are right.

3. Jun 21, 2015

### micromass

Staff Emeritus
The reasoning Griffiths gives is indeed incorrect, but the result is still true: summing deviations from the average gives you zero.

4. Jun 21, 2015

### WWGD

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.

5. Jun 22, 2015

### Alexandre

On the idealized curve yes, on the actual data no. Besides it's a joke.

6. Jun 22, 2015

### WWGD

Ah, sorry, I did not get it the 1st time. I guess a Carlin quote should have made it clear.

7. Jun 22, 2015

### Alexandre

http://cdn.alltheragefaces.com/img/faces/large/happy-yes-l.png [Broken]

Last edited by a moderator: May 7, 2017