The perception of ensemble characteristics is often regarded as an antidote to an established bottleneck in focused attention and working memory, both of which appear to be limited in capacity to a few objects only. In order to test the associative law of summation, observers were asked to estimate the mean size of four circles relative to a reference circle. When there was no time to scrutinize each individual circle, observers discriminated the mean size difference identically, irrespective of whether the same summary size increment or decrement was added to or subtracted from the size of only one, two, or all four circles. Since observers judged the size of individual circles, the position of which was indicated after they were displayed, considerably less accurately than the mean size of the four circles, it is very unlikely that explicit knowledge of the size of the individual elements is the basis of mean size judgments. The sizes of individual elements were pooled together in an obligatory manner before size information had reached awareness. The processing of size information seems to be largely constrained to only one measure at a time, with a preference for mean size rather than the individual measures from which it is assembled. (C) 2014 Elsevier Ltd. All rights reserved.