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Let $\overline{X} = \frac{1}{n} \sum_{i=1}^n X_i$, and $S^2 = \frac{1}{n-1} \sum_{i=1}^n (X_i - \overline{X})^2$.

It is well known that if the $X_i$ are IID normals, say $X_i \sim N(\mu, \sigma)$, then $\frac{\overline{X} - \mu}{S / \sqrt{n}}$ is $t$-distributed with $n-1$ degrees of freedom. In turn, as $n$ goes to infinity, this will converge in distribution to a $N(0, 1)$ variable.

I have two questions (I have not tried to prove the statements, and couldn't easily find precise references):

1) Is it true that if the $X_i$ are IID (with mean $\mu$, say, and finite variance), not necessarily normal, then $\frac{\overline{X} - \mu}{S / \sqrt{n}}$ converges in distribution to a $N(0, 1)$ variable?

2) Is it true that if $X_i$ are independent, not necessarily identically distributed or normal (but with some suitable growth bound on the variances), then $\frac{\overline{X} - \frac{1}{n} \sum_{i=1}^n E[X_i]}{S / \sqrt{n}}$ converges in distribution to a $N(0, 1)$ variable?


Seems like the answer to the first question is here: Is there a theorem that says that $\sqrt{n}\frac{\bar{X} - \mu}{S}$ converges in distribution to a normal as $n$ goes to infinity?

I suppose I can arrive at the same conclusion for the second question by imposing Lyapunov's central limit theorem.

Christian
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    These questions make no sense, because "$n$" cannot possibly appear in any formula for what the sequence converges to. – whuber Dec 28 '16 at 19:57
  • The allegedly nonsensical part or the question can be split in two questions: Does $\frac{\overline{X} - \mu}{S / \sqrt{n}}$ converge to normal? When is the distribution of $\frac{\overline{X} - \mu}{S / \sqrt{n}}$ well approximated by a t-student? In fact, the whole question seems equivalent to "When can we use t-test with a non normal variable?" which makes a lot of sense. – Pere Dec 28 '16 at 20:07
  • Agreed, @whuber. I re-formulated the question to what I really need to know. Thanks. – Christian Dec 28 '16 at 21:12
  • You might want to consider coupling Slutsky's theorem with whichever version of the [CLT](https://en.wikipedia.org/wiki/Central_limit_theorem) will work for known variance (or set of variances) as suited for your question. Write the appropriate fraction as a CLT-relevant term in the numerator, and a $s/\sigma$ type term in the denominator, apply Slutsky to the ratio X/Y where Y -> 1 and apply CLT.to X. Just be careful to set it up correctly for the different variances version – Glen_b Jan 16 '17 at 06:15

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