Which type of variation is NOT typically associated with improvisation?

Choral variation is typically not associated with improvisation because it refers to a structured form of variation that is often composed and written for a group of voices or instruments, adhering to a specific harmonic framework. In choral variations, the variations are usually planned ahead of time and involve intricate arrangements that do not leave room for spontaneous interpretation or on-the-spot creativity, which characterizes improvisation.

In contrast, thematic, rhythmic, and melodic variations can all be aspects of improvisation as they allow musicians to take a theme or basic idea and transform it through free expression. Thematic variation involves altering the main theme in various ways; rhythmic variation focuses on changing the rhythm while keeping the melodic content; melodic variation would involve altering the melody itself while maintaining the underlying chord structure. These types of variations are often integral to improvisational practices in music, where musicians create new and unique interpretations in real-time.