sentences:  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.Thus, unless participants are drawn from a narrow range of skill-levels, their optimal ranges will be mutually exclusive, and observations at a single, controlled tempo will be likely to result in significant contamination of eye movement data.Most studies "have" sought to compare the skilled and the unskilled in the hope of generating pedagogically useful data; aside from Smith (1988), in which tempo itself was an independent variable, Polanka (1995), who analysed only data from silent preparatory readings, and Souter (2001), who observed only the highly skilled, none has set out to control tempo strictly.