Estimates have been made by the Norwegian Petroleum Directorate (NPD) of different categories contributing to potential oil and gas resources for the Norwegian Continental Shelf. The estimates for each sub-area: North Sea, Norwegian Sea, Barents Sea, are evaluated using probabilistic methods to determine both the most likely values of potential reserves (or resources) in a given category, and also to assess the range of uncertainty of each category, i.e. the volatility of each estimate.