Mass provision of healthcare through a digital medium can be greatly enhanced by the use of serious games. The accessibility and engagement provided by a serious game to the subject can significantly increase participation. The commercial games industry employs numerous techniques to analyse soft data collected from early users of an application to evolve the application itself and improve the experience of playing it. A game for mass stereoacuity testing of young children is used as a case study in this paper, to illustrate how soft feedback can be used to improve the effectiveness of a clinical trial. The key to the approach is identified as rapid incremental evolution of the application and trial protocol in a manner which increases the amount and usefulness of soft data collected, and reacts to issues identified in the soft data in a timely fashion. It is hoped that the approach can be adopted for a wide range of digital applications for mass health provision.