The Reliability of Fits’s Law as a Movement Model for People with and without Limited Fine Motor Function Ather Sharif Victoria Pao asharif@cs.washington.edu paov@uw.edu Paul G. Allen School of Computer Science & Engineering, Department of Psychology, University of Washington University of Washington Seattle, Washington Seattle, Washington Katharina Reinecke Jacob O. Wobbrock reinecke@cs.washington.edu wobbrock@uw.edu Paul G. Allen School of Computer Science & Engineering, The Information School, University of Washington University of Washington Seattle, Washington Seattle, Washington Figure 1: (left) Participant using a mouse to perform reciprocal pointing tasks. (right) Screen shot of a 1-D reciprocal pointing task from the FitsStudy program [68] showing two vertical ribbon targets. The starting target is highlighted in blue. A label with the text "Start Here" is shown indicating where to begin the series of pointing trials. ABSTRACT For over six decades, Fitts’s law (1954) has been utilized by re- searchers to quantify human pointing performance in terms of łthroughput,ž a combined speed-accuracy measure of aimed move- ment efciency. Throughput measurements are commonly used to evaluate pointing techniques and devices, helping to inform soft- ware and hardware developments. Although Fitts’s law has been used extensively in HCI and beyond, its test-retest reliability, both Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proft or commercial advantage and that copies bear this notice and the full citation on the frst page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specifc permission and/or a fee. Request permissions from permissions@acm.org. ASSETS ’20, October 26–28, 2020, Virtual Event, Greece © 2020 Association for Computing Machinery. ACM ISBN 978-1-4503-7103-2/20/10. . . $15.00 https://doi.org/10.1145/3373625.3416999 in terms of throughput and model ft, from one session to the next, is still unexplored. Additionally, despite the fact that prior work has shown that Fitts’s law provides good model fts, with Pearson correlation coefcients commonly at r =.90 or above, the model ft- ness of Fitts’s law has not been thoroughly investigated for people who exhibit limited fne motor function in their dominant hand. To fll these gaps, we conducted a study with 21 participants with limited fne motor function and 34 participants without such limita- tions. Each participant performed a classic reciprocal pointing task comprising vertical ribbons in a 1-D layout in two sessions, which were at least four hours and at most 48 hours apart. Our fndings indicate that the throughput values between the two sessions were statistically signifcantly diferent, both for people with and without limited fne motor function, suggesting that Fitts’s law provides low test-retest reliability. Importantly, the test-retest reliability of Fitts’s throughput metric was 4.7% lower for people with limited fne motor function. Additionally, we found that the model ftness