Inter-examiner and intratesting variability can be induced by a considerable number of factors, such as variation on the size, brightness, and complexity of the target; speed of target presentation; type of stimulus (i.e., flat fusion vs stereoscopic); methods of presentation (i.e., Risley prism, anaglyph, or
vectograph) or of changing the stimulus parameters; and variations in lighting, as well as the detail of instructions and the level of motivational support provided to the patient during the evaluation [30,42,50].
They may be helped by appropriate therapeutic intervention, namely small steps of disparity stimuli, as well as smooth and continuous disparity ramp stimuli (e.g.,
vectograph ramp stimuli) per models of the vergence system [22,31].