Review of statistical methods used in quantifying NDT reliability

This paper reviews a number of statistical methods used to quantify the reliability of flaw detection and sizing by NDT from experimental data. False calls are also an important aspect of inspection reliability, but these will not be considered in this paper.

For flaw detection, different methods are applicable according to whether the outcome of a trial is recorded as: □

a binary variable, ie hit/miss data (typical of enhanced visual techniques such as MPI), or □

a continuous variable, ie a signal amplitude relative to a given threshold (typical of ultrasonics or eddy current testing).

In the former case, the method of analysis is further divided into methods that group the data, and methods that treat it as a whole to calculate a probability of detection (POD) curve. In any event there are restrictions on the data set that can make this method cumbersome for experimental application. The second method (called ‘response versus size’ or â versus a) requires the signal amplitude and a threshold for detection. The POD is then produced from a set of data that contains more information than the hit/miss method, and this can allow a smaller number of flaws to be used.

For flaw sizing, attention is usually focused on the amount of undersizing that can be allowed, which can then be used to set realistic acceptance criteria. However, it may also be important to quantify the amount of any oversizing, since this can cause unnecessary repairs or plant shutdown.

Examples of the use of each method are given, and specific applications carried out recently by TWI to support the inspection reliability.