Just wondering if anyone is familiar with clustering nominal inputs. I've been looking at SOM as a solution but apparently it only works with numerical features. Are there any extensions for categorical features? Specifically I was wondering about 'Days of the Week' as a possible features. Of course it is possible to convert it into a numerical feature (i.e. Mon - Sun corresponding to nos 1-7) however then the Euclidean distance between Sun and Mon (1&7) would not be the same as the distance from Mon to Tues (1&2). Any suggestions or ideas would be much appreciated.
Commonly nominal variables are dummy coded when used in SOM (e.g., one variable for with a 1 for Monday 0 for not Monday, another for Tuesday, etc.).
You can incorporate additional information by creating combined categories of adjacent days. For example: Monday&Tuesday, Tuesday&Wednesday, etc. However, if your data relates to human behaviour it is often more useful to use Weekday and Weekend as categories.