1. A computer-implemented method of controlling and monitoring a drilling operation by determining a set of one or more cases in case-based reasoning (CBR), the method comprising: receiving a data stream comprising information on a monitored situation; using a computer, generating a plurality of parallel data streams from the received data stream, each of the parallel data streams containing at least one parameter contained in the data stream; using a computer, generating an overall similarity score for each of the parallel data streams by comparing each parallel data stream to a different one of a plurality of cases; and determining a set of one or more cases in dependence on the generated overall similarity scores; wherein each of said overall similarity scores is generated by a different one of a plurality of comparison agents and each of the comparison agents receives one of the plurality of parallel data streams; and wherein each comparison agent generating one of said overall similarity scores by: receiving one of said plurality of parallel data streams; generating a further plurality of parallel data streams, wherein each of the generated further plurality of parallel data streams is dependent on said received one of the plurality of parallel data streams by the comparison agent; generating, for each of said generated further plurality of parallel data streams, a similarity score in dependence on a comparison between information on a feature in the generated further parallel data stream and stored information on the feature of a case, wherein each of the similarity scores of said generated further plurality of parallel data streams is generated in dependence on a comparison with stored information on a different feature of the same case; generating an overall similarity score between the received one of the plurality of parallel data streams and the case in dependence on the generated similarity scores; and controlling the drilling operation based on the generated similarity scores.