摘要：This research aims at producing a CAT software that uses the triangle decision tree method in the test item selection procedure and detecting the CAT ability in estimating the test-takers’ ability accurately and correctly. This research used the research and development approach (R&D). The data were collected through observation, documentation, inquiry, and the data were analyzed descriptively and quantitatively. The findings are as follows. (1) The CAT developed is:based on users’ need, web-based, user-friendly, interactive, highly secured, and easily accessible. (2) The CAT can recognize three different users: school administrators, teachers, and students. (3) The CAT software is able to provide adaptive items according to students’ answers. Overall, the CAT software can perform the tasks well to select test items and to measure test-takers’ability. It can be seen from correlation value between ability estimation result (θ) in CAT with nilai ulangan murni (NUM) students at school which is high enough t that is 0, 67.Keywords: the triangle decision tree method, maximum likelihood method
