While contribution analysis provides a step-by-step approach to verify whether and why an intervention is a contributory factor to development impact, most contribution analysis studies do not quantify the ‘share of contribution’ that can be attributed to a particular support intervention. Commissioners of evaluations, however, often want to understand the size or importance of a contribution, not least for accountability purposes. The easy (and not necessarily incorrect) response to this question would be to say that it is impossible to do so. However, in this CDI Practice Paper written by Giel Ton, John Mayne, Thomas Delahais, Jonny Morell, Barbara Befani, Marina Apgar and Peter O’Flynn, we explore how contribution analysis can be stretched so that it can give some sense of the importance of a contribution in a quantitative manner. The first part of the paper introduces the approach of contribution analysis and presents ideas to capture the change process in theories of change and system maps. The second part presents research design elements that include ranking or quantitative measures of impact in the verification of the theory of change and resulting contribution story.