The NHS is introducing a new “payment by results” system of financial flows. Crucial to the reforms are both the tariffs for each activity and how activity is measured at the trust level. Activity will be based on hospital episode statistics (HES), a complex dataset containing over 12 million records per year on NHS patient care in England. The basic record is a finished consultant episode of care (the time spent under the care of one consultant). An admission, or spell, is defined as a continuous period of time spent as a patient within a trust, and may include more than one episode. Using HES data, we contrasted spell based activity and episode based activity, using myocardial infarction as an example.