Size averaging of irrelevant stimuli cannot be prevented.

Research suggests that subjects can compute the mean size of two sets of interspersed objects concurrently, but that doing so incurs a cost of dividing attention across the two sets. Alternatively, costs may arise from failing to exclude irrelevant items from the calculation of mean size. Here, we examined whether attention can be selectively deployed to prevent the inclusion of items from an irrelevant, concurrently displayed set in the computation of the relevant set's mean size. The results suggest that mean size is computed prior to the deployment of attention, failing to exclude processing of items that are irrelevant to the task. The influence of the irrelevant items is evident both with brief exposures of the set (200ms) and in a simultaneous judgment task with unlimited viewing time, suggesting that attention cannot be effectively deployed to facilitate selective averaging of the size of the relevant set. Size averaging appears to precede the deployment of selective attention, suggesting that it may be carried out automatically, without intention.