Our aim is to understand the improvements in patient outcomes brought about by having more nurses present in the hospital ward. Nurses are central to delivering higher quality patient care. The UK – as many other countries -  faces a shortage of nurses: vacancies are at a record level and recruitment is expensive and difficult. It is thus vital that this scarce and valuable resource is used in a way that most benefits patients. Nursing teams are composed of individuals with different qualifications and levels of seniority. Which types of nursing staff matter most for improvements in patient outcomes? Does these effects vary across different types of ward or different types of patient? Could we improve patient outcomes by using staff in a different way: for example, by ensuring that individuals who are familiar with each other work more frequently with each other?