Acceptability may be regarded in a three-tier framework. At the top end of the risk spectrum are those risks which are clearly intolerable and must be designed out, irrespective of cost. At the opposite end of the spectrum are risks so low that they may be regarded as negligible and justify no further attention. In between lies a region in which risks are not intolerable but neither are they negligible and in which further reduction may be justified to render the final risk level As Low As Reasonably Practicable (ALARP). Cost-benefit calculations may form the basis for deciding if such further reductions represent justifiable use of additional resources. In this type of study, the reduction in risk produced by a proposed safety measure is compared with the cost of implementation.