This paper is focused on an innovation which involved students being assessed on short written responses to an issue to be addressed in the following classroom session. The innovation was evaluated through a student survey, individual and focus-group interviews with participants, and the analysis of a critical friend. Positive findings included promoting student preparation and reflection, and enabling the teacher to understand students' prior knowledge before the next session. Challenges included some student misperceptions about the assessment innovation, and the difficulty of assessing the short responses efficiently and reliably. The analysis suggests that starting small is a useful principle for assessment change. The discussion also brings out some wider issues of incentives and barriers for assessment innovation, and sketches some related future research directions.