Use samples to estimate standard deviation

Question

The followings are independent samples from two normal populations. Both of which have the same standard deviation.

16, 17, 19, 20, 18 and 3, 4, 8

How to estimate standard deviation?

Answer 1

You could compute $\frac{1}{n} \sum_{i=1}^n (x_i -\overline{x})^2$ for both samples and then take the average.

Answer 2

Given independent samples of size $n_1$, sample variance $s_1^2$, and $n_2$ with sample variance $s_2^2$, the pooled estimate of the common variance is $$ s^2=\frac{(n_1-1)s_1^2+(n_2-1)s_2^2}{n_1+n_2-2} $$