Trials are one of the best ways of testing treatments, but they can be expensive and time consuming. The amount of data collected has a big influence on both cost and time. We aim to understand how much time trial teams spend collecting the most important trial data (called primary outcomes) compared to the other data they collect (secondary outcomes). Outcomes are things like pain, blood pressure, or weight. Small-scale work suggests that trial teams spend most of their time on the less important outcomes. Our proposed large-scale work will find out whether this is correct. We also want to understand the time taken to collect core outcome sets–an agreed minimum amount of information–compared with trials that do not use them to see if they improve efficiency or worsen it. Once we have the above, we will speak with trial teams and others involved in trials to understand what will help them to plan and fund their work more efficiently and also to develop guidance trial teams can use in the future. We hope our results will make it more likely that time isn’t given to less important outcomes at the expense of the most important.