Abstract: This present disclosure relates to computer-implemented methods for use in electronic fraud detection systems, and to data processing apparatus, computer programs, and computer readable storage media for performing the same. One such computer-implemented method comprises accessing a data structure comprising a plurality of transaction records; and executing one or more decision processes sharing a common ruleset, each decision process being configured to use the ruleset to output fraud decisions for transaction records, wherein a fraud decision for a transaction record is a prediction based on the ruleset as to whether the transaction to which the transaction record relates is fraudulent; wherein for each decision process, executing the decision process comprises: reading one or more transaction records for a predetermined historical period from the data structure; and determining, using the ruleset, fraud decisions for the one or more transaction records read from the data structure.