A model is developed for representing computations that arise when deterministic algorithms process workloads whose detailed structure is uncertain. The new computational model, which is based upon the notion of loosely constrained deterministic systems (LCD systems), incorporates a characterization of uncertainty that is expressed entirely in terms of observable properties of the computation itself. This provides a number of advantages over traditional probabilistic models. In addition to providing a more direct link to problems that arise in practical settings, LCD models offer a new perspective on the analysis of risk and support a new method for improving the efficiency of certain computer driven simulations.