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Title: Outlier labeling with boxplot procedures
Authors: Sim, C.H.
Gan, F.F. 
Chang, T.C.
Keywords: Exploratory data analysis
Older statistics
Outlier region
Tolerance limits
Issue Date: Jun-2005
Citation: Sim, C.H., Gan, F.F., Chang, T.C. (2005-06). Outlier labeling with boxplot procedures. Journal of the American Statistical Association 100 (470) : 642-652. ScholarBank@NUS Repository.
Abstract: In this article we focus on the detection of possible outliers based on the widely used boxplot procedures. The outliers in a set of data are defined to be a subset of observations that appear to be inconsistent with the remaining observations. We identify the outliers by constructing a boxplot with its lower fence (LF) and upper fence (UF) either (a) satisfying the requirement that if the given sample is outlier-free, then the probability that one or more of the sample data would fall outside the region (LF, UF) is equal to a prescribed small value α, or (b) taken to be the tolerance limits, derived from an outlier-free random sample, within which a specified large proportion β of the sampled population would be asserted to fall with a given large probability γ. Exact expressions that can be routinely used to evaluate the constants needed in the construction of the boxplot's outlier region for samples taken from the family of location-scale distributions are obtained for both procedures. This article shows that the commonly constructed boxplot is in general inappropriate for detecting outliers in the normal and especially the exponential samples. We recommend that the graphical boxplot be constructed based on the knowledge of the underlying distribution of the dataset and by condoling the risk of labeling regular observations as outliers. © 2005 American Statistical Association.
Source Title: Journal of the American Statistical Association
ISSN: 01621459
DOI: 10.1198/016214504000001466
Appears in Collections:Staff Publications

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