Case study
To illustrate the strengths and limitations of different approaches to model specification we use data from a recent study evaluating the impact of the 2012 Health and Social Care Act in England on hospital admissions and outpatient specialist visits [10]. This policy aimed to involve general practitioners (GPs) in commissioning (planning and purchasing) secondary care through the establishment of GP-led Clinical Commissioning Groups. GP-led commissioning is expected to reduce health care costs by shifting care away from secondary care to primary and community settings [11]. We therefore hypothesized that the reforms would result in a relative reduction in secondary care activity (inpatient admissions and outpatient visits). The health and social care act was enacted in April 2012, there was then a 12 month period during which the Clinical Commissioning Groups worked alongside the existing health care administrative bodies before taking over fully independent commissioning in April 2013. We had quarterly data on all NHS hospital admissions and outpatient visits between the second quarter of 2007 and the final quarter of 2015. More details about the intervention and the data can be found in the original study [10].