1. A method of screening transactions, the method comprising: receiving, at a computer, first data characterizing a first transaction; storing, by the computer, the first transaction in a transaction database that includes a first plurality of transactions each associated with a probability of fraud; determining, by the computer, a first probability that the first transaction is fraudulent based on the first data and the probability of fraud of one or more transactions in the transaction database; setting, by the computer, the probability of fraud for the first transaction in the transaction database to the first probability; 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 based on the transactions in the transaction database, a probability density function with respect to time for receiving the chargeback; determining, by the computer, an area of a portion of the probability density function corresponding to an amount of time since acceptance of the first transaction; determining, by the computer, a second probability that the first transaction is fraudulent based on the first probability and the area; updating, by the computer, the probability of fraud for the first transaction in the transaction database to the second probability; 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 updated probability of fraud of the first transaction in the transaction database.