Extension neglect is neither universal nor absolute . When extension information is both salient and readily interpretable an additive extension effect is observed : the effects of the valuation of the prototype and of the size of the relevant set are additive . This pattern violates normative rules that require non -linear combination of the two types of information. In the situations we have discussed, the relevance of extension may be obvious if the quantity mentioned in the problem is readily classified as high or low. Under such circumstances, responses will show some sensitivity to extension. For example, even naive respondents will appreciate that an annual death rate of .0004% from chlorinated water is very low, because of the impressively large number of leading zeros. However, there are situations in which the quantitative information is less easily interpreted: unless the two numbers are seen together, for example, the subjective difference between two large quantities such as 20,000 or 200,000 birds dying in oil ponds is not very impressive (Hsee, 1996). These are the conditions under which complete neglect of scope may be observed