Outlier

An outlier is an atypical value that does not belong to the distribution of the rest of the values in the data set.

Values that are more than 2.5 standard deviations from the mean can be defined as outliers. This would hold when the distribution is unimodal and symmetrical.

For skewed data, median is a better indicator of central location than mean. In such a case, an observation can be considered as an outlier if it more than 1.5 inter-quartile range away from the closest quartile. An outlier would be labeled an extreme outlier if it is more than 3 inter-quartile range from the closest quartile, and it would be called mild otherwise.

See Also:
median, mean

References:

Devore, J. and Peck, R. "Summarizing a Data Set: Boxplots." §4.3 in Statistics: The Exploration and Analysis of Data Duxbury, Thomson Learning, pp. 111, 2001.

Frank, H. and Althoen, S.C. "Outliers." §A.1 in Statistics: Concepts and Applications Cambridge, Great Britain: Cambridge University Press, pp. 142-143, 1995.

Outlier Detection in Multivariate Analytical Chemical Data, Egan, W. J., Morgan, S. L., Anal. Chem,. 1998, 70, 2372-2379.

http://mathworld.wolfram.com/Outlier.html

http://en.wikipedia.org/wiki/Outlier

Cite This As:

Dogra, Shaillay K., "Outlier" From QSARWorld--A Strand Life Sciences Web Resource.
http://www.qsarworld.com/qsar-statistics-outlier.php