We hypothesize the
movement endpoint distribution as a sum of two
independent normal distributions. One distribution reflects
the relative precision governed by the speed-accuracy
tradeoff rule in the human motor system, and the other
captures the absolute precision of finger touch independent
of the speed-accuracy tradeoff effect. Based on this
hypothesis, we derived the FFitts model—an expansion of
Fitts’ law for finger touch input. We present three
experiments in 1D target acquisition, 2D target acquisition
and touchscreen keyboard typing tasks respectively. The
results showed that FFitts law is more accurate than Fitts’
law in modeling finger input on touchscreens. At 0.91 or a
greater R
2
value, FFitts’ index of difficulty is able to
account for significantly more variance than conventional
Fitts’ index of difficulty based on either a nominal target
width or an effective target width in all the three
experiments.