Effect of posture on target acquisition with a trackball and touch screen

Computing devices are becoming more ubiquitous and are increasingly being used in unconventional environments and body postures. For example, more commonly computers are being used while standing up or walking slowly, such as with bank teller machines or hand-held devices. This paper investigates the effect of posture on Fitts´ law and cursor position time in user interfaces when using a touch screen or trackball as the input device. Several experiments are described that measure the effect of standing and walking slowly during the interaction. Fitts´ law was found to be an effective predictor of movement time across various targets sizes. Standing and walking did not have a significant negative effect on movement time when using a stylus, although a significant increase in error rate was discovered. However, when using a trackball in a standing posture, movement time and error rate both showed a significant increase. Based on these findings, touch screens appear to be more effective input devices for interactive systems that are used in non-sitting postures