In science education, laboratory practicals are frequently assessed through submission of a report. A large increase in student numbers necessitated us adapting a traditional practical report into an online test with automated marking. The assessment was designed to retain positive features of the traditional laboratory report but with added pedagogic and administrative benefits made possible through the online medium. After performing their experiments, students were given idealised data, enabling immediate comparison with their own results, and asked to perform a series of calculations based on that ideal data. The two-part test asked questions about the students' calculations. Part I was formative, ensuring that the students had mastered basic concepts before advanced concepts were tested in Part II. The test rewarded correct methodology and understanding as well as the right answers and gave absolute consistency to the marking scheme. Students could submit at their convenience and received instant feedback. The assessment was met with emphatically positive feedback from students. In addition, it was possible to track submissions by students, providing insights into their behaviour. Students appeared to group into three submission styles--early submitters, considered submitters and last minute submitters--information which can be useful to guide pedagogic practice. In the latest iteration, analysis of submission behaviour gave us confidence to reduce the time before the submission deadline, which resulted in a substantial increase in student attainment.