 Many controversies on risk assessment concern the balance between risks of type I and type II errors. Whereas science gives higher priority to avoiding type I errors than to avoiding type II errors, the balance can shift when errors have practical consequences. This can be seen from a case in which it is uncertain whether there is a serious defect in an airplane engine. A type II error, i.e., acting as if there were no such a defect when there is one, would in this case be counted as more serious than a type I error, i.e., acting as if there were such a defect when there is none. (The distinction between type I and type II errors depends on the delimitation of the effect under study. In discussions of risk, this delimitation is mostly uncontroversial. Lemons et al. 1997; van den Belt and Gremmen 2002.)