The UK has over 30 major longitudinal research studies, with about 2.2million people taking part. There are potential research benefits of understanding the extend to which people take in multiple studies. These benefits are scientific and pragmatic. Scientifically, it is important that researchers understand measurement error; which is a factor of all longitudinal studies no matter how well they are managed. By comparing like information collected by different studies on the same person we can help understand how the different study methods impact on quality and error. Pragmatically there may be cost benefits from being able to share information (e.g. expensively generated genetic information) and participant benefits (e.g. reducing burden of taking part). Furthermore, information collected in one study may help inform strategies to deal with information missing in other studies. However, these sorts of comparisons are relatively new and the ethics, governance and information security considerations need to be thought through carefully to ensure that data sharing and analysis are conducted in an appropriate manner which is acceptable to participants and those running the studies.