Are spell strenghts linear or exponential?

I got a question for the community. Are spell strenghts linear or exponential? Or is it different for some spells?
Example:
Coc 80 gives 80% skillboost and coc100 gives 100%
How about Aura Of Shared Pain? Does 80 return only 20% less than 100? How about Frostbrand etc?

Thanks for your thoughts and experience guys.