How does Viscovery handle missing values?
Data records with missing values or invalid entries are recognized by Viscovery and treated appropriately in the analysis. For numerical attributes, all entries that are not numbers will be treated as missing. For nominal text attributes, all values that you did not define will be treated as missing. The basic operation with a SOM is to look up the best-matching node. If an input data record is not complete (has missing values), then the look-up is limited to the available values. That is, the SOM is treated as if the nodes were shorter vectors (in math speak: the SOM is projected into the data space that consists of the available values) and then the lookup is conducted in this reduced map. This happens for each individual record. It is possible to substitute missing values with the lookup values of the matching nodes. When a data mart is exported, the missing attribute values of the data mart records can even be replaced by the node values of the corresponding nodes in a SOM.
