[Haskell-cafe] Terminology: different levels of strictness

If I consider the functions head, length, elem & sum, each is of them is strict, as: head/length/elem x/sum _|_ are always _|_. However: head (x:_|_) is never _|_. length [_|_, _|_, _|_ ...] is also never _|_. elem x [4,5,6,8,2,90,_|_,_|_ ...] is *only sometimes *_|_ (depending on x value). In fact, only sum [4,5,6,8,2,90,_|_,_|_ ...] is always _|_. Which shows they don't have the same level of strictness. So can you say things like "all these functions are strict, but some are *more *than other", or "sum is *deeply strict*" ...? What terms can you use to compare those functions?