A common question I have heard is, “How do you conduct usability testing with mobile applications?” What generally follows is a discussion around the benefits of testing in a usability lab or in the field.
In our practice, we have found that the best use of time and resources is to have a usability analyst conduct a heuristic or expert review of the initial design or sketches. In a heuristic review, the usability analyst compares an application or prototype with established design principles to determine what works well and what needs improvement. 
Following the expert review, we employ a Rapid Iterative Testing and Evaluation (RITE) testing protocol. With RITE, we take wireframes or rough sketches of the application and have users step through the proposed tasks. At this point in the process, we are trying to establish the overall workflow and high level navigation. In addition, studies have affirmed the use of mobile simulations via a personal computer (PC) during the early stages of the design cycle. Later on, the interaction, aesthetics, and functions can be tested on the device itself in the lab and finally in the field (Ballard, 2007; Nielsen, 2006; Weiss, 2002).
Overall, lab testing is best for early to mid-design cycle testing. Project teams should consider field testing late in the design cycle when most obvious issues have been addressed (especially when environmental issues are critical). Be aware that field testing can introduce other issues.
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References
Kaikkonen, A. Kekäläinen, M. Cankar, T. Kallio, and A. Kankainen. Usability testing of mobile applications: A comparison between laboratory and field testing. Journal of Usability Studies, 1(1):4–17, 2005.
C.M. Nielsen et al (2006). It’s Worth the Hassle! The Added Value of Evaluating the Usability of Mobile Systems in the Field, NordCHI 2006.
B. Ballard, Designing the Mobile User Experience. John Wiley & Sons, Ltd. (2007).
S. Weiss, Handheld Usability, John Wiley & Sons, Hoboken, NJ (2002).
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Is your project team interested in learning how usability can improve the user experience of your mobile applications? Please contact us at info at improvedusability.com.