Objectives: Improvement of service quality and security is required in any business area in society. The purposes of this chapter are to identify and to verify our study hypotheses that cognitive activities, work environment and organizational climate/culture are highly related with human performance reliability and that human performance reliability was predicted by organizational control mode. This chapter will also emphasize that it is important to focus on the implication of latent variables perceived for tacit knowledge as well as articulate knowledge in knowledge management. Methods: The subjects surveyed in the case study are 356 clinical nurses and healthcare providers working in a national hospital in Japan. The questionnaires used were prepared by referring to the methodologies developed by Hollnagel et al. for assessing human reliability. Results: The score of improved reliability in strategic organizational control mode was the highest, while the one in scrambled mode was the lowest among four control modes of organization. Performance reliability was significantly influenced by organizational climate and work environment as well as cognitive activities of the participants. This was the similar trend observed in industry. In concluding, the latent factors, i.e., the variables in the genotype embedded deep in a complex organization, were the determinants for predicting human performance reliability in this case study. These results suggested that the variables in the genotype representing cognitive activities, nursing work environment and organizational safety climate were important factors as well as the variables in the phenotype which were observable.