This paper presents the development of human factors assessment tool for safety-critical industries. The assessment tool is a questionnaire about human factors issues that can cause human error and accidents in consequence. It is based on the 'Dirty Dozen' human error concept of Dupont (1997, as cited in Safety Regulation Group, 2002). The advantage of this concept is the adaptable nature in the applied field. Moreover, it was intended to search for a latent structure behind the dirty dozen concept. Hence, a questionnaire with 240 items was tested and optimised in two large metal manufacturing companies. In order to find a hidden structure behind the dirty dozen, a factor analysis of all twelve scales was performed ...