I like how Math Academy does it:

- each lesson or review or quiz has a target number of XP

- if you do it perfectly, you get the target XP plus 2

- if you do it almost perfectly, you get the target XP

- if you do poorly, you get fewer XP, and can even get negative XP

This last eliminates any incentive to guess or rush (at least in the cases I've directly observed).

(author here). Agreed - definitely like how Math Academy does it. That said, my kids definitely _do not_ like earning negative XP. I tend to agree with my kids on that one :)