Funny thing I've found in David Griffiths QM textbook

[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.

 

[mathman]

You are right.

 

[micromass]

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

 

[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.

 

[Alexandre]

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.

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

 

[WWGD]

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

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

 

[Alexandre]

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

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