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Fitts’ law has proven to be a strong predictor of pointingperformance under a wide range of conditions. However, ithas been insufficient in modeling small-target acquisitionwith finger-touch based input on screens. We propose adual-distribution hypothesis to interpret the distribution ofthe endpoints in finger touch input. We hypothesize themovement endpoint distribution as a sum of twoindependent normal distributions. One distribution reflectsthe relative precision governed by the speed-accuracytradeoff rule in the human motor system, and the other captures the absolute precision of finger touch independentof the speed-accuracy tradeoff effect. Based on thishypothesis, we derived the FFitts model—an expansion ofFitts’ law for finger touch input. We present threeexperiments in 1D target acquisition, 2D target acquisitionand touchscreen keyboard typing tasks respectively. Theresults showed that FFitts law is more accurate than Fitts’law in modeling finger input on touchscreens. At 0.91 or agreater R2 value, FFitts’ index of difficulty is able toaccount for significantly more variance than conventionalFitts’ index of difficulty based on either a nominal targetwidth or an effective target width in all the threeexperiments.
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