Motor delay

It also takes time for a user to make a physical movement. In most handheld computers, users respond by touching some target: either physical key(s) or user interface objects on the touch-screen. We only consider the usage of user interface objects. The Fitts Law [6] states that for a normal human being, the motor time to carry out a movement is related to the size of the target and the distance to the target as

$$MT= 0.23 +0.166\cdot\log_2(\frac{A}{W}+1)(s)\vspace*{-2mm}$$

where $A$ is the amplitude of the movement (assumed to be one quarter of the screen height in this work) and $W$ the width of the target as measured in the direction of motion [18]. We have adopted the values of coefficients from [18].

The above three models are together referred to as the psychological model in this work. Perceptual and motor delays can be predicted relatively well. However, the cognitive delay can be only pessimistically predicted based on the Hick-Hyman Law, as mentioned before. Therefore, the psychological model is used to predict the lower bound on user delay. In the next section, we propose a history-based user delay model to predict the actual delay.