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(2012) considers that an outlier is a scientific term describing "things or phenomena that lie outside normal experience“. With a plain explanation, (2015) defines an outlier is "an observation which deviates so much from the other observations as to cause the suspicions that it was generated by a different mechanism."
In terms of probability distribution, (n.d) indicates that an outlier is a number that is more than 1.5 times the length of the data set away from either the lower quartile (Q1) or upper quartiles (Q3). As such, if a number is less than Q1 - 1.5 x IQR or greater than Q3 + 1.5 x IQR, then it is an outlier.
Similarly regarding the notion of outliers within boxplots, Hamilton (2013) focuses on observations of more than 1.5 of IQR that beyond the first or third quartile are plotted as individual points.
In an statement, the College of Saint Benedict Saint John's University (n.d.) says that "not uncommonly real datasets will display surprisingly high maximums or surprisingly low minimums called outliers".
Why do Outliers matter?
Outliers are important because they might bring in crucial information for the adjustment of decision making; particularly in circumstances where the outliers are genuine. It is believed that the genuine outliers often indicate an extreme of behaviour of the process under study, and therefore, before starting on any formal analysis, all outliers must be examined carefully (Unknown author, n.d.). This author also warns that every single outlier should not routinely be removed without further justification.
References
College of Saint Benedict Saint John's University (n.d.). Box Plot: Display of Distribution. Retrieved 22/2/2016
from http://www.physics.csbsju.edu/stats/box2.html
Hamilton, L.C. (2013). Statistics with STATA: Updated for Version 12.Boston, MA: BOOKS/COLE CENGAGE Learning
Unknown author (n.d.). Outliers. Retrieved 22/4/2016 from https://www.google.com.au/search?safe=strict&espv=2&biw=1440&bih=775&site=webhp&tbm=isch&sa=1&q=why+outliers+important&oq=why+outliers+important&gs_l=img.12...0.0.2.1143004.0.0.0.0.0.0.0.0..0.0....0...1c..64.img..0.0.0.ZNMkTEy4h6c&dpr=1&cad=cbv&bvch=u&sei=m7kZV9XoKIrQjwPIzLb4Dw#imgrc=XvhoHcEmdLSCdM%3A
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