We report one teacher's response to a top-down shift from external examinations to internal teacher assessment for summative purposes in the Republic of Ireland. The teacher adopted a comparative judgement approach to the assessment of secondary students' understanding of a chemistry experiment. The aims of the research were to investigate whether comparative judgement can produce assessment outcomes that are valid and reliable without producing undue workload for the teachers involved. Comparative judgement outcomes correlated as expected both with test marks and with existing student achievement data, supporting the validity of the approach. Further analysis suggested that teacher judgement privileged scientific understanding, whereas marking privileged factual recall. The estimated reliability of the outcome was acceptably high, but comparative judgement was notably more time-consuming than marking. We consider how validity and efficiency might be improved and the contributions that comparative judgement might offer to summative assessment, moderation of teacher assessment and peer assessment.