In measurement data there may exist *outliers*, data items lying
very far from the main body of the data. The outliers may result, for
instance, from measurement errors or typing errors made while
inserting the statistics into a data base. In such cases it would be
desirable that the outliers would not affect the result of the
analysis. This is indeed the case for map displays generated by the
SOM algorithm: each outlier affects only one map unit and its
neighborhood, while the rest of the display may still be used for
inspecting the rest of the data. Furthermore, the outliers can be
easily detected based on the clustering display: the input space
is, by definition, very sparsely populated near the outliers. If
desired, the outliers can then be discarded and the analysis can be
continued with the rest of the data set.

It is also possible that the outliers are not erroneous but that some data items really are strikingly different from the rest. In any case the map display reveals the outliers, whereby they can either be discarded or paid special attention to.

Mon Mar 31 23:43:35 EET DST 1997