It is not always Y N A. Sometimes ther are some votes for some candidates. I would prefer:
```
PERCENT_BASE_CHOICES = (
    ('WITHOUT_INVALID', ugettext_lazy('only all valid votes')),
    ('WITH_INVALID', ugettext_lazy('all votes casts including invalid votes')),
    ('DISABLED', ugettext_lazy('Disabled (no percents)')))
```