A transaction authorization system comprises an authorization engine (18) configured to perform an analysis in relation to a received request for authorization of a current electronic payment transaction and generate an authorization output based on the analysis, and a database (22) storing a plurality of transaction records for past electronic payment transactions. The authorization engine is configured to: access the database to determine one or more distribution functions based on at least some of the stored transaction records, determine one or more intermediate values in relation to the received authorization request based on the one or more distribution functions, determine a conditional probability in relation to the one or more intermediate values based on at least some of the stored transaction records, the conditional probability indicative of a probability of a predetermined transaction result under the condition of the one or more intermediate values, and generate the authorization output based on the determined conditional probability.