Encyclopedia > Outlier

  Article Content


In statistics, an outlier is a single observation far away from the rest of the data.

One definition of "far away" in this context is:

less than Q1 − 1.5 × IQR or greater than Q3 + 1.5 × IQR
where Q1 and Q3 are the first and third quartiles, respectively, and IQR is the interquartile range (equal to Q3Q1).

These values define the so-called inner fences, beyond which an observation would be labeled a mild outlier.

Extreme outliers are observations that are beyond the outer fences:

less than Q1 − 3 × IQR or greater than Q3 + 3 × IQR

In the case of normally distributed data, using the above definitions, only about 1 in 150 observations will be a mild outlier and only about 1 in 425,000 an extreme outlier.

Outliers usually demand special attention since they may indicate problems in sampling or data collection or transcription. Alternatively, an outlier could be the result of, for example, a truly unusual response to a given treatment, calling for further investigation by the researcher.

See also: box plot

All Wikipedia text is available under the terms of the GNU Free Documentation License

  Search Encyclopedia

Search over one million articles, find something about almost anything!
  Featured Article
Canadian Charter of Rights and Freedoms

... rights, but it was only applicable to the federal government and was not considered part of the Constitution of Canada like the Charter. Links Canadian Charter of ...

This page was created in 31.8 ms