An important concept in statistics is measures of spread or variation. A basic numerical description of a data set requires both measures of central tendency and spread. For example, say that you know that the average, or mean, income in a certain area is $30,000. Taking this number alone, it could just as easily describe an area where incomes are spread out fairly evenly between $25,000 and $35,000, as an area where most of the incomes are $10,000 and a few exceptionally hard-working and/or very lucky people have an income of $1,000,000. This is where measures of spread come in.

Just as there are several measures (mean, median, mode) used to measure central tendency, there are also several measures of spread, including:

- Range, the difference between the lowest and highest values in a sample,
- Quartiles, representing values where 25%, 50%, or 75% of the data are below that value
- Variance, which describes how great a spread exists in the data
- Standard deviation, which describes the spread of data in relation to the mean.