Interrupted time series (ITS) study is a kind of before-and-after controlled study. In most cases, it is used to retrospectively analyze the effect of a certain intervention. By analyzing the changes of some outcome indicators before and after the implementation of the intervention and the differences before and after the intervention, the actual effect of the intervention is judged. It is especially suitable for evaluating the changes in group-level outcomes caused by certain interventions in a specific time, such as the implementation of medical reform policies and the changes in the diagnosis and treatment guidelines of certain diseases.