Interpretative - interpretive

AWE cannot do better than quote the on-line version of Modern English Usage, s.v. 'interpretative':

interpretative, interpretive. The preferred form for this word meaning ‘serving to interpret or explain’ is interpretative, on the analogy of authoritative, qualitative, and quantitative:

You may be wondering why I am rabbiting on about interpretative processes when the theme of this article is how to build a bracket clock — Practical Woodworking, 1990.

However, interpretive is about twice as common, perhaps reinforced by the natural tendency to articulate both words in this way in rapid speech:

Chinese culture has undergone major interpretive phases in recent decades — Dædalus, 1991.