Estimating frequency and size of effects due to experimental manipulations in eye movement research

In studies of eye movement behavior, once it has been demonstrated that an experimental manipulation has produced a reliable effect, it is often useful to try to estimate the frequency with which the effect occurred. This paper describes the Frequency of Effects Analysis and illustrates its use with data from a study on characteristics of the perceptual span of adult readers. The results of the analysis indicated that, in one instance, a manipulation which produced a 21 msec increase in fixation duration was actually producing a 151 msec increase in only 21% of the instances, and was having no effect in the remaining 79% of the cases.