Median (statistics)

A median is a measure of central tendency used frequently in statistics.

Median of a data set

The median of a finite set of real numbers $\{X_1, ..., X_k\}$ is defined to be $X_{(\frac{k+1}2)}$ when $k$ is odd and $\frac{X_{(\frac{k}2)} + X_{(\frac{k}2 + 1)}}2$ when $k$ is even, where $X_{(i)}, i \in \{1,...,k\}$ denotes the $k^{th}$ order statistic. For example, the median of the set $\{2, 3, 5, 7, 11, 13, 17\}$ is 7.

Median of a distribution

Median of a discrete distribution

If $F$ is a discrete distribution, whose support is a subset of a countable set ${x_1, x_2, x_3, ...}$ , with $x_i < x_{i+1}$ for all positive integers $i$ , the median of $F$ is said to lie between $x_i$ and $x_{i+1}$ iff $F(x_i)\leq\frac12$ and $F(x_{i+1})\geq\frac12$ . If $F(x_i)=\frac12$ for some $i$ , $x_i$ is defined to be the median of $F$ .

Median of a continuous distribution

If $F$ is a continuous distribution, whose support is a subset of the real numbers, the median of $F$ is defined to be the $x$ such that $F(x)=\frac12$ . Clearly, if $F$ has a density $f$ , this is equivalent to saying $\int^x_{-\infty}f = \frac12$ .