Abstract: Roughly described, a data mining system for use in evolving individuals based on data samples in a training database. The individuals have a plurality of conditions and, for each of a plurality of the data items in a sample, output an action which depends upon application of the individual's conditions to the data item. The conditions include a state of the individual, and the actions include at least one which affects the state of the individual if asserted. Each candidate individual further has a fitness estimate which the system updates in dependence upon the testing results. Testing involves applying the conditions of the individual to data items of the samples to assert actions, and for a subset of at least one but less than all of the data items to which the conditions are applied, modifying the asserted action for the data items in the subset.