1. A computer-implemented method, the method comprising: obtaining a plurality of representations of predictive models, each representation having been received from a different client wherein each representation is associated with a respective user and comprises a description of a respective predictive model; selecting a model implementation from a plurality of model implementations for each of the obtained representations based on a status of the representation's respective user, the model implementation comprising one or more computer programs operable to execute on one or more computing devices; and associating each of one or more of the model implementations with a node in a directed graph wherein for one or more ordered pairs of nodes in the graph the prediction output of a model implementation associated with a first node in the pair serves as input to a model implementation associated with a second node in the pair.