Zomara, A. M. M., Hamad, T. A., Amin, M. A. (2019). 'THE EFFECT OF SIX SIGMA METHODOLOGY USAGE ON RAISING EFFICIENCY OF HUMAN RESOURCES' PERFORMANCE AND COST REDUCTION IN WORK ENVIRONMENT IN AIRPORTS SECTOR', Journal of Environmental Science, 48(3), pp. 455-484. doi: 10.21608/jes.2019.163892