This paper proposes a first attempt to study quantitatively Collective Free Improvisation (CFI). We report an experiment designed to study the relationship between the improvisers’ individual high-level decisions and the improvisation’s form perceived by external auditors. We recorded 16 trios in which the improvisers used a MIDI-pedal to indicate in real-time a significant change in their own musical production. Expert listeners were later asked to segment each improvisation so that we obtained their perception of the structure. By analysing the correlations between musicians’ individual decisions and listeners’ segmentation points, we discuss how the improvisation’s form emerges from the improvisers’ individual behaviours. While the overall structure depends on individual contributions and decisions which can be well-identified, it is never fully determined by them.