Outlier

An outlier is a data point that is significantly different from the other values in the data set. It can be significantly larger or significantly smaller than the others.

Categorical data sometimes also have unusual elements; these can be called outliers as well.

However, it is best to use the term outlier when applying a mathematical process to identify it. This approach helps differentiate between an intuitive and a more formal approach.

Extra

Explaining "Significantly Different"

What significantly different means can depend on many things. For numerical data, the following definition is one of the several approaches that can be used. A data value can be considered an outlier if it is farther away from the closest quartile than a certain multiple of the interquartile range.

• The multiplier 1.5 is commonly used to identify outliers. This value was suggested by the esteemed American mathematician John Tukey.
• The multiplier 3 is sometimes used to identify extreme outliers.

The diagram below shows a box and whisker plot of a data set. Move the slider to see which data point is an outlier according to the description above.