Mean, Median, Variance and Standard Deviation are simple and frequently used concepts in statistics as well as in every branch of mathematics. It is also a popular term in machine learning and deep learning for performance and accuracy meaasurement.

Mean

Mean is the average value considering all the values in a set of data. It is calculated by the total sum divided by the number of elements. The mean is usually denoted by $\bar{x}$

$\bar{x}=\frac{1}{n}\sum_{i=1}^{n}x$

where x is the value and n is the total number of elements.

Median

The mean is a pretty good measurement technique for understanding the average value. However, in case of outlier value, mean is not the perfect measurement criteria. It is the middle value. It is calculated by summing up middle value (for even number series) or just dividing by two to get the middle number (for odd number series).

Variance
$s^2=\frac{1}{n-1}\sum_{i=1}^{n}{(x-\bar{x})}^2$

Standard Deviation

$\sigma=\sqrt{\frac{1}{n-1}\sum_{i=1}^{n}{(x-\bar{x})}^2}$