CAATs (Computer Assisted Auditing Techniques) are used to test application controls as well as perform substantive tests on sample items. Following are the types of CAATs: Generalized Audit Software (GAS): It allows the auditor to perform tests on computer files and databases. Custom Audit Software (CAS): It is generally written by auditors for specific audit tasks. CAS is necessary when the organization's computer system is not compatible with the auditor's GAS or when the auditor wants to conduct some testing that may not be possible with the GAS. Test Data: The auditor uses test data for testing the application controls in the client's computer programs. The auditor includes simulated valid and invalid test data, used to test the accuracy of the computer system's operations. This technique can be used to check data validation controls and error detection routines, processing logic controls, and arithmetic calculations, to name a few. Parallel Simulation: The auditor must construct a computer simulation that mimics the client's production programs. Integrated Test Facility: The auditor enters test data along with actual data in a normal application run.