Why search for target absence is so slow (and careful!): the more targets there are, the more likely you are to miss one.

Five factors were tested as possible explanations of why it takes longer to find the critical row that does not contain a target letter (search for absence) than the critical row that does contain a target letter (search for presence). Various stratagems (e.g., asking subjects to search for a letter not in the memory set, presenting one row at a time and requiring a response to each row, and examination of sequential effects) led to the rejection of Factor 1 (Slower Processing of Targets), Factor 2 (Slower Recovery from Targets), Factor 3 (Spatial Chunking or Use of Peripheral Vision), and Factor 4 ( Temporal Chunking). The evidence favored Factor 5 (Opportunity for Misses): Targets, which as a rule are easy to miss, are relatively more numerous in search for absence, so to avoid making many more errors, the subject must search more slowly (and carefully!). To estimate more accurately the slowing of search, the effect of self-termination (Factor 6), which speeds the search for absence, was removed. The slowing of search in search for absence was found to involve nontarget letters as well as target letters and to involve the memory-comparison stage.