Outlier

Math Glossary

A resource for WE & Friends Mathematics and Science Projects

Home  | A  | B  | C  |  D  | E  | F  | G  | H  |  I  | J  | K  | L  | M  | N  | O  | P  | Q  | R  | S  | T  | U  | V  | W  | X  | Y  | Z

This glossary is far from complete. We are constantly adding math terms.
For instructions on adding new terms, please refer to Math Glossary Main Page

Definition

Outlier A rare, atypical observation. A data value that is numerically distant from the rest of the data. The presence of an outlier may indicate some sort of problem -- possibly a data error or a data value which is fundamentally different from the other data.

Tip: Identifying outliers...

There is no universally-agreed-on criterion for defining an outlier, however using [math]1.5\, [/math] times the interquartile range (IQR) to identify data values substantially below the first quartile (Q1) or above the third quartile (Q3) provides a good rule of thumb. With this rule an observation is considered a suspected outlier if it is:

• below [math]\ Q1 - 1.5(IQR)\, [/math]
• above [math]\ Q3 + 1.5(IQR)\, [/math]

Regardless of your method for identifying outliers, always assess the effects of outliers on the statistical conclusions.

Examples

The histogram shown below displays a dataset with one high outlier - a value between 75 and 80, with the next highest value between 50 and 55.

Web Resources

Hayden, Robert W. (2005). "A Dataset that is 44% Outliers." Journal of Statistics Education. 13(1). High, Robin. (2000). Dealing with 'Outliers:' How to Maintain Your Data's Integrity. Computing News. University of Oregon.

.