Every technology, especially in ubiquitous computing area, should be geared to improve the quality of their users' life ultimately. In order to achieve this goal, the important pre-requisite is to measure the contribution of the technology to quality of life reliably and validly. This study provides a theoretical as well as an empirical basis for the development of better measures for the contribution of a mobile data service technology to the quality of its users' lives. The reliability and validity of the proposed metrics were verified through online surveys in Korea and Japan. The survey results also indicated that critical mobile data services could be identified based on the degree of contribution to the quality of life of mobile data-service users. Moreover, the critical services were found to be different between Korea and Japan. (c) 2006 PICMET.