If you run a rehabilitation programme for people who have committed criminal offences, how do you decide whether it is successful? It seems a simple question, but suppose that you are an organisation with limited resources, working with a specific group of individuals. How can you find the data to compare them to a similar group who have not been through your programme? How can you be confident that the results are not down to chance? In this article, Mark Purver and Helen Williams, from the Ministry of Justice, describe how the Justice Data Lab can provide the necessary expertise to help these organisations measure their success.