In a mobile app that I am developing I use multiple measurements - user acceleration, ambient noise level, consistency of the speed of user interactions with on-screen information etc - to keep track of how the user is probably feeling.
For each of the above I have found good literature that describes research in the area. For instance, the first derivative of user acceleration (jerk) has been shown to correlate well to how tense the user is feeling. That apart I have incorporated machine learning techniques that ensure that the assessments that keep improving with usage.
However, there is one additional measure that I would like to incorporate - if only I could find at least some preliminary research that shows the right way forward. Both iOS and Android devices report the X and Y radii (major and minor radii) of the elliptical area that is tapped when the user interacts with the screen. One possibility, for instance, would be to use consecutive touch events to deduce a numeric measure in the form of a "haptic jerk" (analogous with jerk as it relates to acceleration). Perhaps haptic jerkiness increases as the user gets more distracted/busy/stressed?
My searches have thus far yielded nothing useful - perhaps because I am not quite using the right search terms. Perhaps someone here is aware of prior published research on the subject?