The use of a test-grid as on aid to test-construction is well known. This paper describes its use in an exercise involving student teachers in a biology teacher education programme. The purpose is two-fold; to serve as a feed-bock instrument and as a teaching aid. Students are required to plan tests on photosynthesis and human metabolism/nutrition under constrained conditions, which force them to examine carefully their priorities. The grids are then analysed and form the basis for individual and group discussions. The diversity of the numbers of items allotted to pre-selected sub-topics on the one hand, and the toxonomic levels of cognitive demand on the other, by individual students, enable the teacher to; a) diagnose whether previous course content has been internalized by the students, and to identify cases where further tutoring is necessary, and b) to use group and individual data to convey certain basic facts about the assessment process. The method described has been successfully used by the author for many years.