The use of technical justification in performance demonstration

Performance demonstration, or qualification as it is also known, of inspect ions involves showing that the inspection procedure, equipment and personne l are capable of achieving the required performance. One way in which this has been attempted in the past is to produce test-pieces replicating the co mponent in question containing deliberately introduced defects and then to apply the inspection. An alternative when suitable samples are available is to use items removed from the plant which have been found to be defective. The problem with both methods is that the number of defects needed to make a statistically significant case is impracticably large. This is firstly b ecause there are many factors influencing the outcome of the inspection and the number of permutations of factors which require investigation is large . Secondly, for any single permutation of defect size, position, type and o rientation, a large number of defects is needed. To establish 95% confidenc e in. 95% reliability requires the detection of 59 defects from a sample of 59. The impossibility of producing highly reliable statistical proof of inspect ion performance has led to an alternative in which other information than t hat arising from practical trials is used in addition, Such information may include results from previously conducted test-piece trials or site inspec tions, the use of mathematical models of the inspection or results from lab oratory work to investigate the influence of the various influential parame ters separately. What is used will depend on what is available in any parti cular case. This assembly of information is known as a Technical Justificat ion and, taken together with the results of rest-piece trials provides a mo re convincing demonstration of inspection capability than the trials by the mselves. Furthermore, the Technical Justification can be used to design the test-pieces used in trials and frequently enables a stronger case to be ma de with fewer test-pieces, thereby reducing the cost of the process as well as making it more effective.