In research, intelligence is tested and results are generally as obtained, however in a clinical setting intelligence may be impaired so estimates are required for comparison with obtained results. Premorbid estimates can be determined through a number of methods, the most common include: comparison of test results to expected achievement levels based on prior education and occupation and the use of hold tests which are based on cognitive faculties which are generally good indicators of intelligence and thought to be more resistant to cognitive damage, e.g. language.