Mobile text entry: relationship between walking speed and text input task difficulty

• Authors: Sachi Mizobuchi;Mark Chignell;David Newton
• Venue: Proceedings of the 7th international conference on Human computer interaction with mobile devices & services
• Year: 2005

The effect of key size on text entry on a handheld device while walking and standing was examined in order to answer the following questions: 1) Will the additional workload of walking amplify the effect of input difficulty? and 2) Can walking speed be used as a secondary task measure of mental workload during mobile text entry? 13 participants (7 males and 6 females) input well known sayings (sentences) in English into a handheld device in each of four size conditions, with the text input box ranging in width between 2 and 5 millimetres (mm). Text input speed increased with larger size of text box up to a size of 3mm, and text input speed was faster when standing (vs. walking). The effect of size did not depend on whether participants were walking or standing. Errors were significantly higher for the 2mm size condition but did not vary for the wider sizes, while subjective ease of input increased with increasing input box width, only crossing the midpoint of the rating scale (i.e., more easy than difficult) at an input box width of 3mm. Based on these results it is recommended that a minimum text input box width of 3mm be used for handheld text input. Walking speed during text entry in this study was relatively low (with a mean of 1.77 km/h) but width of text input box had no additional effect on walking speed over and above the general slowing caused by text entry. Thus the answers to both of the main questions posed in this study were in the negative, although the fact that people had to enter text slowed walking speed by a fixed amount (independent of level of input difficulty) that varied between individuals. Implications for measuring workload in mobile text entry tasks are discussed.