Why is my index not being used?

Why is my index not being used?

1. where surname != 'FRASER'
2. where upper(surname) = 'FRASER'
3. where surname is null
4. where surname like '%FRASER'
5. small tables
6. histograms:
   
   90% rows 'Yes'
   1% rows 'No'
   1% 'Maybe'
   1% 'Not in'
   1% 'Later'
   where answer = 'Yes'

----> IN is faster than EXISTS for subqueries that return few rows.
----> EXISTS is faster than IN for subqueries that return many rows.