The disclosed embodiments relate to a system for tracking and analyzing user interactions with a mobile user interface (UI). During operation, the system collects data while a user interacts with the mobile UI through a touchscreen, wherein the data includes tap-attempt data that specifies coordinates for touchscreen locations that the user has tapped while interacting with the mobile UI. Next, the system compares the tap-attempt data with locations of tappable elements in the mobile UI, wherein each tappable element has an associated tap target size. For each tappable element, the system uses results of the comparison to determine a percentage of tap attempts that have succeeded or failed for the associated tap target size. Finally, the system makes the percentage information available to a UI designer to enable the UI designer to adjust tap target sizes for the mobile UI.