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Depending on the disease or condition in question, the precise mechanisms for determining which individuals are eligible for the protocol will vary considerably. The protocol should delineate with care the inclusion and exclusion criteria. For example, a general clinic may see many potential patients of a particular type, who will then need to be screened to determine whether they are truly eligible for the trial. In other situations, a clinic may be more specialised in nature so that everyone coming to the clinic has been pre‐screened and is likely to satisfy the requirements of the protocol. The proportion of eligible subjects will vary from trial‐to‐trial. In Protocol UKW3 (1992), the number of operable children with renal (Wilms’) tumours was likely to be less than half of those presenting with the disease.