1. A method of screening transactions, the method comprising: receiving, at a computer, first data characterizing a first transaction; determining, by the computer, a first probability that the first transaction is fraudulent based on the first data; receiving, at the computer, second data characterizing a second transaction occurring in a period of time during which a chargeback for the first transaction can be received; determining, by the computer, a second probability that the first transaction is fraudulent based on the first probability and an amount of time since acceptance of the first transaction; and determining, by the computer, a third probability that the second transaction is fraudulent based at least in part on the second data and the second probability.