Based on many years of research here are the formal rules for creating categories.
Each Category MUST be:
1. Mutually exclusive (i.e only one memeber per group)
2. Collectively exhaustive (i.e. all members have a group)
3. Stable and objective (i.e. groups don’t change)
and each category should:
4. Have a consitent level of extraction per level of hierarchy
5. Have ONE classification principle per level of hierarchy
6. Have a clearly delineated topic area, theme or knowledge domain
7. Have names that are self explanatory and informative
8. Contain prototype members that are easy to recognise
9. Contain all members without becoming too large to navigate
10. Be easily understood by the envisaged users
Reference:
Eppler, MJ 2008, “A process based classification of knowledge maps and application examples”, Knowledge and Process Management, Vol 15, no. 1, pp 59-71.