# Properties of Sample Mean

If we have sequence of random variables $$X_1, X_2, ... X_n$$, we can define Sample Mean $$\bar X$$ as their average:

$$\bar X = {1 \over n}(X_1 + X_2 + ... + X_n)$$

The mean and variance of $$\bar X$$ are easily calculated by using basic theorems.

So, let $$X_1, X_2, ... X_n$$ be a random sample from a distribution with mean μ and variance $$σ^2$$ and $$\bar X$$ - sample mean. Then $$E(\bar X) = μ$$ and $$Var(\bar X) = σ^2 / n$$.

Proof: If we know for Y = aX + b, we have E(Y) = aE(X) + b and $$E(X_1 + ... + X_n) = E(X_1) + ... + E(X_n)$$, it follows that:

$$E(\bar X) = {1 \over n}\sum_{i=1}^n E(X_i) = {1 \over n} * nμ = μ.$$

Furthermore, since these random variables are independent and we know for Y = aX + b, we have $$Var(X) = a^2Var(X)$$ and $$Var(X_1 + ... + X_n) = Var(X_1) + ... + Var(X_n)$$, it follows that:

$$Var(\bar X) = {1 \over n^2}Var(\sum_{i=1}^n X_i)$$

$$= {1 \over n^2}\sum_{i=1}^n Var(X_i) = {1 \over n^2} * nσ^2 = {σ^2 \over n}.$$

In words, the mean of $$\bar X$$ is equal to the mean of the distribution from which the random sample was drawn, but the variance of $$\bar X$$ is only $${1 \over n}$$ times the variance of that distribution. It follows that the probability distribution of $$\bar X$$ will be more concentrated around the mean value than was the original one. In other words, the sample mean $$\bar X$$ is more likely to be close to the mean than is the value of just a single observation $$\bar X$$ from the given distribution.

By applying Chebyshev inequality to $$\bar X$$, we get:

$$P(|\bar X - μ| \ge t) \le {σ^2 \over nt^2}.$$