The European Committee for Future Accelerators [ECFA] and the High Energy and Particle Physics Division of the European Physical Society [EPS-HEPP] have jointly prepared a document to help professional committees to assess the achievements of individual particle physicists from large experimental collaborations. The document is motivated by the need to explain to scientists from outside the community of experimental high-energy physics the way these collaborations, of few hundred to few thousand members, are organized in order to design, build and carry out the scientific program of large-scale experiments.