From 1995, data on students' achievement in schools (i.e., teacher's grades) and all data on achievement in the 5-subject group certificate--the "Matura" exam--have been systematically gathered for the entire yearly cohort of students in upper secondary education in Slovenia. This paper describes an on-line data selection system and data analysis tool designed for national subject testing committees, schools, and teachers as feedback for analyses of effectiveness, for the improvement of teaching in the classroom and for better learning. The use of data to influence teacher planning of future instructional activities for new groups of students (or for adjustment of policies) can in the long run lead to systematic improvement. This might be considered a secondary level of formative assessment--a key step in a continuous PDCA (Plan, Do, Check, Act) process to establish and assure the quality of education. (Contains 14 figures and 2 tables.)