Multimodality During Fixation – Part II: Application to Spatial Precision-Related Distributions
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This paper is a follow-on to our earlier paper (Friedman, Lohr, Hanson, & Komogortsev, 2021), which focused on the multimodality of angular offsets. This paper applies the same analysis to the measurement of precision. One typical measure of the spatial precision of an eye-tracking device is the standard deviation (SD) of the position signals (horizontal and vertical) during a fixation. The SD is a highly interpretable measure of spread if the underlying error distribution is unimodal and normal. However, in the context of an underlying multimodal distribution, the SD is less interpretable. We will present evidence that the majority of such distributions are multimodal. Only 21% to 23% of position distributions were unimodal. On the other hand, 68% to 70% were strongly multimodal. We present an alternative method for measuring precision that is appropriate for both unimodal and multimodal distributions. This alternative method produces precision estimates that are substantially smaller than the traditional approach, which ignores multimodality. Although it is our impression that both the presence of drift or the presence of a microsaccade predisposes toward multimodality, we have not formally examined this at this time. We present illustrations of both unimodality and multimodality with either drift or a microsaccade present during fixation.