In this report, NPC finds that providing better access to offending data through a 'Justice Data Lab' would have huge benefits. The government's recent introduction of payment by results contracts means charities are under renewed pressure to prove their impact on things like reoffending. But the lab would not just be about payment by results -- it would have three other major benefits for charities: * It would help charities to prove their impact on reoffending, by showing how their users' offending behaviour changes over time, and assessing whether reductions in offending were due to the charity's support; * It would help charities to improve their impact, for instance by identifying groups that continue to reoffend in spite of support, and may need more attention; and * It would help charities to identify what works, by analysing the impact different services have on offending to identify which is most effective at reducing reoffending. In an era where government cuts to services are commonplace, the potential benefits of knowing what works are significant. If the Justice Data Lab proves a success, the concept could be adopted by other government departments in the UK and abroad. This could mean providing access to data in areas such as drug use, mental health, and housing, where there is a real need to improve our knowledge of what works. Establishing a Justice Data Lab will be complex and certainly will not happen overnight. However, we are heartened by the commitment of the Ministry of Justice and the contributions of the charities that have been involved so far. We also appreciate the tenacity of NPC in supporting the Ministry of Justice to take this challenging work forward. We are optimistic that the Justice Data Lab can be turned from concept into reality, bringing us one step nearer to the long term transformation in data sharing and improving services that we all want to see.