1. An apparatus to assist with constructing data describing a Conditional Probability Table, comprising: a display device to display a representation of a set of states, each one of the states being associated with a corresponding variable, the states being ordered in a hierarchy based on a selected significance level by a user, wherein at least one of the states displayed is designated as being a most significant state for the corresponding variable; an input device to receive an input representing a probability estimate to be associated with said displayed state; and a component to use the inputted probability estimates to generate data describing a Conditional Probability Table; wherein each variable represents a node having a plurality of potential states which are ordered based on the selected significance level by the user, and wherein the hierarchy includes nodes positioned in a three-layer structure, which includes a context layer, a hypotheses and alternative hypotheses layer, and an evidence layer, so that influence works downwards and inference works upwards, and wherein the state of the context nodes influences the state of the hypothesis nodes and the alternative hypothesis nodes, which in turn influence the state of the evidence nodes, and wherein the state of the hypothesis nodes and the alternative hypothesis nodes is inferred from the state of the evidence nodes, and the state of the evidence nodes is inferred from the hypothesis nodes and the alternative hypothesis nodes.