What is the question?: impact of question length and illustration support on the success and skip rates of preuniversity mock online tests

Online quizzes are a powerful educational tool that allow learners to self-evaluate their knowledge. This paper analyzes the results of over 20,000 pre-university mock online quizzes in the technical drawing subject of the Spanish University Admission Test to assess the impact of (a) the presence of accompanying graphic illustrations and (b) the length of the questions on the questions success and skip rates. The performance difference of the students in two tests 6 years apart and participating in two different intakes is also discussed. The results show that questions with image support receive more successful answers and are skipped less often, while increasing question length have the opposite effect. These findings should help improving the design of online quizzes, including more visual content and/or rephrasing the questions to be more concise.