Living examples: the positive effect of instructional videos on physics tests in Dutch pre-university classes. Aim of this study is to measure the effect of instructional videos on physics tests, taken in the highest classes of Dutch pre-university education (vwo 5 and 6) in 2014. The students (aged between 17 and 19) were randomly assigned to experimental and control groups. In a first trial, the control group used a textbook to prepare for a physics test, while the experimental group watched instructional videos on YouTube. In a second trial, the control group watched the videos and the experimental group used the textbook to prepare for a second physics test. In this research design, both between and within group comparisons of the test results have been made. Each and every comparison indicated a significant and relevant positive effect: if instead of a textbook, videos were used for preparation, the percentage of correct answers improved with 10.5 points [90% CI 7.5 - 13.5] and the average percentile score increased with 24 points [90% CI 17 - 30], with an overall effect size (Cohen's D) of 0.9. This indicates a strong positive influence on tests results when short instructional videos are being used in class.