This paper compares timing and key-velocity data collected from a skilled performance of Schubert's G♭-major Impromptu (Opus 90) with a number of performances generated by a version of a musical expression algorithm proposed by Todd (1992). Regression analysis is used to demonstrate both the shortcomings of this model as a complete explanation of musical expression and how it might be more successfully used as a tool for analyzing data from real performances. Used in this second manner, the algorithm is shown to provide a general expressive baseline against which other aspects of expression may be highlighted. It is also suggested that such a baseline provides a method of decomposing performances into continuous and discrete forms of expression. It is concluded that using algorithmic models as heuristic tools, rather than as explanations in themselves, may better serve our increased understanding of the flexible and multiple nature of musical expression.