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Abstract The present study investigates the impact of different key solutions of mobile phones on users' effectiveness and efficiency using the devices. In the first experiment, 36 children (9 – 14 years) and in the second experiment 45 young adults (19 – 33 years) completed four common phone tasks twice consecutively on three simulated phones that had identical menus, but different key solutions. An approach was undertaken to quantify the complexity of keys in three models, incorporating different factors contributing to the keys' complexity (number of key options, number of modes and number of modes with a semantically dissimilar meaning), in order to predict users' performance decrements. As a further main factor, the degree of the users' locus of control (LOC) was measured and interactions with performance outcomes were studied. As dependent measures, the number of inefficient keystrokes, the number of tasks solved and the processing time were determined. Results showed a significant effect of control key solutions on users' efficiency and effectiveness for both children and young adults. Moreover, children's LOC values significantly interacted with performance: children with low LOC values showed the lowest performance and no learnability, especially when using keys with a high complexity. From the three factors contributing to the complexity of keys, keys exerting different functions with semantically inconsistent meanings had the worst effect on performance. It is concluded that in mobile user interface design keys with semantically inconsistent meanings should be generally avoided. Keywords: Mobile phonesKey complexityModesCognitive compatibilityUsabilityMenu navigation performanceChildren Acknowledgements We would like to acknowledge the valuable contribution of Alexander Schwade, who collected and analysed the children data. Sadly, he passed away before this paper was concluded. His inspiration will never be forgotten. Moreover, we thank Philipp Brauner for programming the software for displaying the mobile phones and logging user actions as well as Sarah Hatfield, Preethy Pappachan and Judith Strenk for their research support. A final thanks is devoted to two anonymous reviewers for their constructive comments on an earlier version of this manuscript.
Ziefle et al. (Tue,) studied this question.