Understanding RSE(Residual Standard Error) I am reading the book " An Introduction to Statistical Learning " and I have trouble understanding their explanation of RSE(Residual Standard Error ) . This is what the book says :
"The RSE is an estimate of the standard deviation of \epsilon . Roughly speaking, it is the average amount that the response will deviate from the true regression line. It is computed using the formula :"

What I don't understand here is definition of RSE as " Standard deviation of \epsilon " .
If Residual = y(i) - yHat(i)
and going by my assumption that \epsilon is also = y(i) - yHat(i)
Isn't the formula of RSE just computing the "root(squared mean)" of \epsilon ?
If \epsilon =  y(i) - yHat(i) ,
Then , standard deviation of \epsilon would be
sum[y(i) - yHat(i) - mean(Y - Yhat)] / n-2 which is not what the above formula does .
So , Technically , I think RSE is just  squared mean of the Residuals or \epsilon and would be wrong to call it " Standard deviation of \epsilon " , if we go by the actual formula for  standard deviation .
Or , In my opinion this should mean the standard deviation of predicted yHat values rather than \epsilon .
So , is this a misuse of the term " standard deviation " in  the book or Am I missing something ?
Please correct or help me understand .
 A: I believe this is because in OLS the average of  residuals is $0$. Hence the formula for standard deviation of residuals ($\epsilon)$ can be given as:
$$\sqrt\frac{\sum (\epsilon_i - \mu_{\epsilon})^2}{n-2}$$
but since in OLS $\mu_{\epsilon}=0$  by construction (see explanation of why this is so in this Mathematics.SE answer) you are left with:
$$\sqrt\frac{\sum (\epsilon_i - 0)^2}{n-2}= \sqrt\frac{\sum (\epsilon_i)^2}{n-2} \implies \sqrt\frac{\sum (y_i - \hat{y_i})^2}{n-2}$$
since $\epsilon_i=y_i - \hat{y_i}$
A: Let's specify that we are speaking about simple linear regression, $y_i=\beta_0+\beta_1x_i+\epsilon_i$.

If Residual = y(i) - yHat(i) and going by my assumption that \epsilon
is also = y(i) - yHat(i)

The error is $\epsilon_i=y_i-\beta_0-\beta_1x_i$, the residual is $e_i=y_i-\hat\beta_0-\hat\beta_1x_i=y_i-\hat{y}_i$. They are not equal.

Isn't the formula of RSE just computing the "root(squared mean)" of
\epsilon ?

You can't compute the root squared mean of $\epsilon$ because you can't observe $\epsilon$. In other words, you don't know, and will never know, $\beta_0$ and $\beta_1$, so you can't calculate $y_i-\beta_0-\beta_1x_i$.

What I don't understand here is definition of RSE as " Standard
deviation of \epsilon " .

Pay attention! Your book says that "The RSE is an estimate of the standard deviation of $\epsilon$": since you can't observe $\epsilon$, the best you can do is using residuals to get an estimate of errors' standard deviation.
