Fancy term for average # Mean (Arithmetic) ## Sample The mean calculated from a subset of the population of size $n$ $ \bar{x}=\frac{1}{n}\sum_{i=1}^nx_i $ ## Population The population mean is the mean of all observations of the population with size $N$ $ \mu=\frac{1}{N}\sum_{i=1}^Nx_i $ Can be skewed by ***outliers*** -- data points that are not representative of the set and influence the mean # Median The middle value of a dataset if it is odd-sized. If the set is even-sized, the median is the average of the two middle values. $ \text{Median}(X)=\begin{cases}X\left(\frac{n}{2}\right)& n \text{ odd}\\\frac{X\left(\frac{n+1}{2}\right)+X\left(\frac{n-1}{2}\right)}{2}&n\text{ even}\end{cases} $ # Mode The most common value in a dataset