Characterising the location of the outer boundary of the outer radiation belt is a key aspect of improving radiation belt models and helps to constrain our understanding of the mechanisms by which the source and seed electron populations are transported into the radiation belts. In this paper, we hypothesise that there are statistical differences in the electron distribution function across the radiation belt outer boundary, and thus analyse electron flux data from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) satellites to identify this location. We validate our hypothesis by using modelled electron L* values to approximately characterise the differences between electron distribution functions inside and outside of the radiation belts. Initially, we perform a simple statistical analysis by studying the radial evolution of the electron distribution functions. This approach does not yield a clear discontinuity, thus highlighting the need for more complex statistical treatment of the data. Subsequently, we employ machine learning (with no dependence on L*) to test a range of candidate outer boundary locations. By analysing the performance of the models at each candidate location, we identify a statistical boundary at ≈8 Earth radii, with results suggesting some variability. This statistical boundary is typically further out than those used in current radiation belt models.