J. Kjeldskov, M.B. Skov, B.S. Als and R.T. Høegh (2004)Is it Worth the Hassle? Exploring the Added Value of Evaluating the Usability of Context-Aware Mobile Systems in the Field(.pdf):

Evaluating the usability of mobile systems raises new concerns and questions, challenging methods for both lab and field evaluations. A recent literature study showed that most mobile HCI research projects apply lab-based evaluations. Nevertheless, several researchers argue in favour of field evaluations as mobile systems are highly context-dependent. However, field-based usability studies are difficult to conduct, time consuming and the added value is unknown. Contributing to this discussion, this paper compares the results produced by a laboratory- and a field-based evaluation of the same context-aware mobile system on their ability to identify usability problems. Six test subjects used the mobile system in a laboratory while another six used the system in the field. The results show that the added value of conducting usability evaluations in the field is very little and that recreating central aspects of the use context in a laboratory setting enables the identification of the same usability problem list.