Is Plum a normative assessment?

Yes. While Plum is predominantly forced-choice, asking for relative, within-person judgments of people about their own tendencies, the resulting scores are then normed in comparison to how much other people in the population make the same relative judgments.

Are normative and ipsative opposites?  

No. This is a common understanding. 
Normative and ipsative are not opposites. 

Research has found that forced-choice (ipsative) assessments, like Plum's, correlate around .70 with likert scale assessments (normative) across a range of traits.  Meaning they have ipsative and normative assessments have an exceptionally strong relationship. This also means that the idea that within-person judgments tell us nothing about between-person trait elevations is false. In other words, if a person is higher on Trait A than B, C, and D, then it significantly increases the odds they are higher than average on A in the population, too.

Feedback and Knowledge Base