Policy fields are usually defined by conventional usage rather than by systematic analysis. This paper experiments with a methodology of boundary analysis that builds inductively through the aggregation of "topics" and policy instruments. Using the case of shipping policy, we develop a list of 473 topics drawn from several bibliographical and policy sources. These are then aggregated into 135 categories with accompanying policy instruments. While the methodology has obvious limitations, the experiment and case study demonstrate that it is possible to systematically estimate the complete possible contents of a policy field. The approach shows sufficient promise to warrant refinements and broader applications to a wide variety of estimation problems. It also makes a modest contribution to the study of policy instruments, which has tended to get stuck at the "classification" stage without much detailed empirical analysis of how and when instrument categories actually apply.
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