Mean, median and mode

One number to describe the center

A measure of center summarizes a whole data set with one typical value.

The mean

From a frequency table, multiply each value by its frequency:

Weighted mean

When values count differently, use a weighted mean: multiply each value by its weight, add, and divide by the total weight.

The median

The mode

The mode is the value that occurs most often. A data set can have one mode, several, or none. It's the only measure of center that works for categorical data.

Outliers: mean versus median

An outlier is a value far from the rest. The mean uses every value, so an outlier pulls it strongly; the median barely moves.

Practice