A transaction authorization system comprises an authorization engine (22) configured to perform an analysis in relation to a received request for authorization of an electronic payment transaction and determine an authorization output based on the analysis, wherein the analysis includes application of one or more checking rules of the if-then clause type comparing one or more transaction-related attributes to one or more predefined attribute threshold values. The system further includes a database system (24) storing transaction records for a plurality of past electronic payment transactions, an evaluation engine (28) configured to access the database system (24) and evaluate a first set of stored transaction records to determine one or more characteristic values related to the first set, and a setting engine (26) configured to access the database system (24) and select a second set of stored transaction records in accordance with at least one of the one or more characteristic values, wherein the setting engine (26) is further configured to evaluate the selected second set of transaction records and set at least one attribute threshold value in at least one checking rule based on the evaluation of the second set of transaction records.