Defects in information technology (IT) services will have greater negative impacts on social and economic activities in the future, since the risks of such defects are increasing due to their wider applications and the growth of IT systems' complexity and scale. In this paper, upon surveying the current state of affairs and seeing that little progress has been made towards understanding and reducing defects in IT services that influence social and economic activities, despite a number of efforts including upgrading software and hardware quality, we propose a new approach to understanding the dependability of IT services by discussing a dependability model. The model consists of not only functions for legacy system dependability but also two types of functions (additional safety and safety operation). Next we evaluate the efficacy of these two types of functions relative to IT dependability, both qualitatively and quantitatively, by using 42 actual cases of IT accidents obtained by the Information-technology Promotion Agency, Japan. Moreover, based on the verification, we examine the ways in which improvement of IT dependability associated with social and economic infrastructure, previously dependent on the skill of individual engineers, is eventually achieved as a result of organizational activities.