Input device matters for measures of behaviour in online experiments.

Studies of perception, cognition, and action increasingly rely on measures derived from the movements of a cursor to investigate how psychological processes unfold over time. This method is one of the most sensitive measures available for remote experiments conducted online, but experimenters have little control over the input device used by participants, typically a mouse or trackpad. These two devices require biomechanically distinct movements to operate, so measures extracted from cursor tracking data may differ between input devices. We investigated this in two online experiments requiring participants to execute goal-directed movements. We identify several measures that are critically influenced by the choice of input device using a kinematic decomposition of the recorded cursor trajectories. Those using a trackpad were slower to acquire targets, mainly attributable to greater times required to initiate movements and click on targets, despite showing greater peak speeds and lower variability in their movements. We believe there is a substantial risk that behavioural disparities caused by the input device used could be misidentified as differences in psychological processes. We urge researchers to collect data on input devices in online experiments and carefully consider and account for the effect they may have on their experimental data.