Outliers, part I: What are outliers?

Our intuition about data tends to lead us to an assumption about that data, namely that the distribution is Normal (that is, Gaussian). If that is not the case, however, say if the data comes from a χ2 distribution, then many intuitive assumptions about the data will be false. χ2 distributions tend to be asymmetric with a long tail extending toward higher values. That long tail means that high values (even compared to the other values in the distribution) of χ2 will be common, and therefore, ipso facto, not unusual or lying at an abnormal distance from the rest of the data. Conversely, since every set of data must have some value that is the largest, as well as one that is the smallest, simply having either of those properties is not enough to flag a datum as an outlier. We will continue our discussion of outliers in a future column of Chemometrics in Spectroscopy coming soon to your mailbox. © 2017 UBM. All rights reserved.