[The PRECIS-2 tool: designing trials that are fit for purpose].

To evaluate the intervention effects of randomized controlled trials (RCT) involved in theoretical efficacy and actual clinical outcome (effectiveness). Pragmatic-Explanatory Continuum Indicator Summary (PRECIS) is a tool to help researchers make decisions in study design which is consistent with the intended purpose of their trial which can be used in the design of RCT to balance the internal validity and external validity. The role of PRECIS has been gradually recognized in the practice of designing clinical trials. To ensure that the design choices are concordant with the intention and the facilitation of use set by patients, clinicians and policy makers, a new PRECIS-2 tool has been developed by mangy international team experts under modification and upgrading the existing PRECIS. The PRECIS-2 tool mainly focuses on trial design choices which determining the applicability of a trial. PRECIS-2 has nine domains, with each of them intends to help the researchers consider the consequences of that design decision in terms of the applicability of the results under particular setting. The purpose of this paper is to introduce the development, basic principle, characteristics and application of PRECIS-2 for the designers and decision makers when working on clinical trials.