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Finger-Fitts law [6] is a variant of Fitts’ law which accounts for the finger ambiguity in touch pointing. In this paper we investigated two research questions related to Finger-Fitts law: (1) Should Finger-Fitts law use nominal target width W or effect target width We to model MT? and (2) should Finger-Fitts law use a pre-defined value (denoted by σa) or a free parameter (denoted by c) to represent the absolute ambiguity caused by finger touch? Our investigation on two touch pointing datasets showed that there are cases where using nominal width has stronger model fitness, and also cases where using effective width is better. Regarding the representation of finger ambiguity, using a free parameter c to represent the ambiguity of finger touch always leads to stronger model fitness than using the pre-defined σa, after controlling for overfitting. It indicates that viewing the finger ambiguity as an empirically determined parameter has more flexibility to capture the ambiguity of finger touch involved in the study. Overall, our research advances the understanding on how to model Finger touch input with Finger-Fitts law. 

Many people with low vision rely on screen-magnifier assistive technology to interact with productivity applications such as word processors, spreadsheets, and presentation software. Despite the importance of these applications, little is known about their usability with respect to low-vision screen-magnifier users. To fill this knowledge gap, we conducted a usability study with 10 low-vision participants having different eye conditions. In this study, we observed that most usability issues were predominantly due to high spatial separation between main edit area and command ribbons on the screen, as well as the wide span grid-layout of command ribbons; these two GUI aspects did not gel with the screen-magnifier interface due to lack of instantaneous WYSIWYG (What You See Is What You Get) feedback after applying commands, given that the participants could only view a portion of the screen at any time. Informed by the study findings, we developed MagPro, an augmentation to productivity applications, which significantly improves usability by not only bringing application commands as close as possible to the user's current viewport focus, but also enabling easy and straightforward exploration of these commands using simple mouse actions. A user study with nine participants revealed that MagPro significantly reduced the time and workload to do routine command-access tasks, compared to using the state-of-the-art screen magnifier.