1. A control process for an expert system implemented with a computer system, wherein said expert system includes an inference network comprising a plurality of inference nodes each representing a predetermined proposition, said inference nodes being respectively coupled to one another in accordance with a plurality of predetermined inference rules to form said inference network, wherein each of said inference nodes is assigned a merit value based on a processing cost for calculating with said computer system a likelihood value of the likelihood of the proposition which said inference node represents, said control process comprising: selecting nodes of said inference network for processing to determine said likelihood values for said nodes in accordance with the respective merit values of said nodes so that those nodes having greater merit values will be processed before nodes having smaller merit values; and limiting the total number of nodes selected for processing in accordance with a predetermined constraint.