A remarkable regularity appears in these experiments: the valuation of the prototype and the extension of the set (base-rate or duration) contribute in strictly additive fashion to the global judgment (see also Anderson, 1996, p. 253). The participants in these experiments appear to reason as follows: ‘‘this medical procedure is quite painful, but it is short’’ or ‘‘this medical procedure is quite painful, and it is also long.’’ In contrast to the logic of global evaluation, which requires multiplicative or quasi-multiplicative effects of extension, the size of the set is used as an extra feature in this reasoning.