It is shown that the use of shell model wave functions in the analysis of quasi-free scattering meets serious difficulties owing to centre-of-mass motion. In contrast, the formalism of the cluster model can be used to construct a suitable wave function in relative coordinates, so that the three-body final state of the quasi-free process can be treated correctly. It is also shown that neglect of anti-symmetrization and core recoil can lead to misinterpretation of the data.