This report focuses on one aspect of calculating impact – attribution. Attribution is an assessment of how much of the outcome was caused by the contribution of other organisations or people. This report aims to provide more practical guidance for organisations who are conducting an SROI. The first section explains where calculating attribution fits into the SROI process. The second section outlines the theory of attribution. The third section gives practical examples of how to estimate attribution. The fourth section opens up some areas for discussion. Section five concludes.