The present invention relates to determining the validity of a transaction of a data processing system such as the validity of a financial transaction carried out at an automatic teller machine (10) which has a magnetic card reader. In addition to the entry of a PIN, a user of the system is requested to enter personal information that has been nominated by the user and may consist of a date of birth and a telephone number. A keypad (16) allows the user to enter the PIN. The user is also requested to enter digits from the personal information to be checked against stored security data so that the level of security can be enhanced. A data processing unit of the system is programmed to cause further requests for digits from the personal information in the event that errors are made in the data entry.