In this article, Ara Massey, Sustainable Design Manager at Slaterpaull Architects, describes how important it is for her to try and understand the impact of early stage design decisions on building performance when designing green buildings. In the absence of better methods, she had to rely on rules of thumb and intuition. However, since Slaterpaull started using Sefaira’s performance based design software, she uses data analysis from the moment she starts working on a project, which has not only changed the way she designs but also tested her intuition.