# How is this question a dupe?

$\mathbb{E}[S_n]$, when $S_n$ takes $X_1+...+X_n$ or $Y_1+...+Y_n$?

I don't think it is the same thing as:

Why does $\mathbb{E}(\frac{X_1+...+X_n}{n})=\mathbb{E}(X)$?

because the former involves conditional expectation.

• I agree it's not a duplicate. When it was originally posted, it appeared to focus on the question the community identified as a duplicate. After it was edited, it more clearly focused on iterated expectation. As it stands, it does appear to duplicate other threads about iterated expectations, such as stats.stackexchange.com/questions/69399. Should we reopen the question and close it as a duplicate of that one? – whuber Dec 23 '15 at 15:17