Almost all studies have compared temporal variables among participants, chiefly the durations of their fixations and saccades. In these cases, it is self-evident that useful comparisons require consistency in performance tempo and accuracy within and between performances. However, most studies have accommodated their participants’ varied performance ability in the reading of the same stimulus, by allowing them to choose their own tempo or by not strictly controlling that tempo. Theoretically, there is a relatively narrow range, referred to here as the ‘optimal range’, in which capacity matches the task at hand; on either side of this range lie the two problematic tempo ranges within which a performer’s capacity is excessive or insufficient, respectively. The location of the boundaries of the optimal range depends on the skill-level of an individual performer and the relative difficulty of reading/performing the stimulus.Souter (2001:80–81)
