Many treatments improve outcomes with few material side effects. They may be expensive, but if they are made available to patients almost all would accept. However, some treatments have more serious side effects, such that a substantial minority of patients would decline. Rationing bodies such as the National Institute for Health and Clinical Excellence treat both scenarios in the same way, calculating an average quality adjusted life year (QALY) for all candidates for the new treatment. Yet people who decline do so because they have different preferences and hence different expectations about the prospect of gain from the treatment. By declining the treatment, they cannot benefit from it. In these situations we argue that considering the QALYs of only patients likely to accept the treatment will lead to a less biased assessment of cost effectiveness.