(...) assessing visual search performance only by reaction time (as is generally done) is limited, because speed of performance depends partially on the participants' willingness to accept erros.
As McElree and Carrasco (1999, p.1532) pointed out, "RT [reaction time] data are of limited value ... because RT can vary with either differences in discriminability, differences in processing speed, or unknown mixtures of the two effects."
As Wolfe (1998, p.56) pointed out:
"In the real world, distractors are very heterogeneous [diverse]. Stimuli exist in
many size scales in a single view. Items are probably defined by conjunctions of many
features. You don't get several hundred trials with the same targets and distractors... A truly satisfying model of visual search will need... to account for the range of real-world visual behaviour."