Variances and Human Factors in Radiographic Image Interpretation
As conventional film radiographic testing continues transitioning to the digital or computed RT process, the POD method of analysis is likely to improve significantly.
All Nondestructive Evaluation (NDE) or examination methods comprise the following steps: the search for, and detection of the discontinuities, the indication or recording or signal–processing of those detected discontinuities, and the human interpretation of that indication, record or signal. Actual discontinuities may not be detected or identified for a number of reasons: the technique is insufficiently sensitive to detect the particular discontinuity, the recording or signal is insufficiently sensitive or clear to allow proper interpretation or the human interpretation step itself fails by not properly discerning an otherwise obvious discontinuity. Any disagreements or variances among qualified NDE inspectors in interpreting NDE (NDT) results therefore reflects, to varying degrees, the extents to which these reasons are applicable in any particular evaluation. In cases where a facility owner and contractor disagree about specific interpretations during a project, there are obvious concerns about project delays and the costs of possible repairs to the alleged discontinuity. Ultimately, all of the ‘missed’ interpretations trace back to inspectors who are either not properly trained or are incorrectly executing the applied technique.
The Probability of Detection (POD) analysis procedure has been developed over the last 40 or so years to address the complete detection-thru-interpretation cycle for various NDE methods. It is possible, with some simple basics of probability, to apply and extrapolate POD analysis to determine the degree of detection variance which might be expected to occur with statistically-independent, qualified NDE inspectors. The study presented here deals primarily with radiographic testing (RT) and RTFIs (Radiographic Testing Film Interpreters) involved in the petroleum industry, but the results should be equally applicable to any number of independent inspectors, in any industry, who interpret results from any NDE method for which POD data are available.