Some of your school notes will mention and explain the definition of unbiased and consistency. But let us just discuss what unbiased means as we do see it quite often in A-level.

What does its mean to say that $s^2$ is an unbiased estimate of $\sigma^2$?

We know that an estimate means its an approximation, meaning that $s^2\ne \sigma^2$. So unbiased will mean that the $E(s^{2})=\sigma^2$. And if we recall expectation is a really intuitive meaning, simply what do you expect? So unbiased indicates that we expect $s^2$ to be $\sigma^2$ but do note that latex s^2\ne \sigma^2\$ still!

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