Any applied statistician who has analysed a number of sets of real data is likely to have come across ‘outliers’. The intuitive definition of an outlier would be ‘an observation which deviates so much from other observations as to arouse suspicions that it was generated by a different mechanism’. An inspection of a sample containing outliers would show up such characteristics as large gaps between ‘outlying’ and ‘inlying’ observations and the deviation between the outliers and the group of inliers, as measured on some suitably standardized scale.