Abstract
Real-world data is that collected outside the constraints of controlled clinical trials and is increasingly informing decision-making in healthcare. The landscape of real-world data in the United Kingdom is set to evolve over the coming months as the government plans to build on databases currently in place by collecting patient data from all family practices and linking this information with hospital records. This initiative, called care.data, has the potential to be an invaluable resource. However, the programme has been criticized on grounds of data privacy, which has led to an extended delay in its implementation and the expectation that a large number of people will opt out. Opt-outs may introduce substantial biases to the dataset, and understanding how to account for these presents a significant challenge for researchers. For the scope and quality of real-world evidence in the United Kingdom to be realised, and for this information to be used effectively, it is essential to address this challenge.