It has been reported in the literature that the disparities of student achievement within schools are often larger than the disparity across schools. In the secondary school context, analysing student achievement data at department level enables a fine-grained approach to identifying particular departments and teachers within them that are enabling students to achieve. This paper describes the steps taken to compare the student assessment results of 41 New Zealand secondary school English, maths and science departments with each other. In some schools results across departments show considerable variation by subject and in other schools results across departments are similar. (Contains 1 table, 6 figures, and 4 notes.)