What are the three levels (I, II, III) NDT of technicians and what does each mean?

There are three levels of NDT of technicians I, II, III, level one being lowest and three the highest. The technician’s placements are based off certifications through employers or the ASNT.  Each of the level of training has different field work hour requirements per method of nondestructive test (NDT), well as written and practical exams for each. We personally train our technicians in house, to the industry standards as to ensure the integrity and reliability as a product.