where expertise comes together - since 1996 -

The Largest Open Access Portal of Nondestructive Testing (NDT)

Conference Proceedings, Articles, News, Exhibition, Forum, Network and more

where expertise comes together
- since 1996 -
332 views
Technical Discussions
Terry Oldberg
Engineering, Mechanical Electrical Nuclear Software
Consultant, USA, Joined Oct 1999, 42

Terry Oldberg

Engineering, Mechanical Electrical Nuclear Software
Consultant,
USA,
Joined Oct 1999
42
00:33 Jun-20-1999
Ethics and Best-Case Analysis

In "Erratic Measure," (Republished in the May issue of NDTnet) Ronald Christensen and I point out that the flaw detection methods of today violate the Unit Measure axiom of probability theory. Ethical questions follow from this, for the violations invalidate value-free methods for estimating the probabilities of error, leaving only value-laden methods.

In designing things, engineers often employ the method of analysis which is worst, from the standpoint of cost but best, from the standpoint of safety. This is called "worst-case analysis." Engineers don't usually employ best-case analysis. However, in estimating the probability of detection (POD), the NDT community employs best-case analysis.

The question of whether to use best-case or worst-case analysis arises when a test is positive for a flaw and the positive relates to more than one flaw. That this relation is one-to-many violates Unit Measure and invalidates probability theory. However, Unit Measure can be preserved and probability theory restored by selecting a flaw and counting it as detected while counting all of the other flaws as not detected. A value judgement is made in selecting the flaw. Thus, this method of analysis is value-laden.

Under best-case analysis, the largest of the flaws is selected. Under worst-case analysis, the smallest of them is selected. Best-case analysis provides an upper bound on the POD of large flaws and a lower bound on the POD of small flaws, while worst-case analysis provides the opposite. Thus, best-case analysis makes NDT look better than it is while worst-case analysis makes it look worse.

The use of best-case analysis without warning is ethically striking under any circumstances but this is particularly true when the organization employing it is a safety regulator. Nonetheless, studies published by the U.S. Nuclear Regulator Commission ("Steam Generator Group Project. Task 13 Final Report: Nondestructive Examination Validation.," NUREG/CR-5185, 1988. U.S. Nuclear Regulatory Commission, Washington, DC.) and the U.S. Federal Aviation Administration ("Reliability Assessment at Airline Inspection Facilities, Violume III: Results of an Eddy Current Inspection Reliability Experiment." DOT/FAA/CT-92/12, III, Federal Aviation Administration, Washington, DC) use best case analysis. Neither agency warns the readers of its report that the POD estimates are the result of best case analysis.

For more than 14 years, I've observed that the NDT community displays no, discernable interest, in modifying its practices to preserve Unit Measure and have always wondered why this is. Here is a theory. If the NDT community were following custom in engineering, by using worst-case analysis in estimating the POD, it could make NDT look better by modifying it to preserve Unit Measure. However, in reality, the NDT community is using best-case analysis and failing to warn people of the departure from custom. It follows that the NDT community would make NDT look worse by modifying it to preserve Unit Measure. Is thiswhy I can't find an interest in preserving Unit Measure?



 
 Reply 
 

Product Spotlight

Sci Aps Z-Series Portable Handheld Analysers

The world’s only handheld analyzer that measures carbon content in stainless (yes even L-grades),s
...
teels, and cast irons. Also accepted for low Si analysis for sulfidic corrosion analysis, and is widely used in the power industry for Cr analysis, for flow accelerated corrosion applications.
>

PAUT Probes

Typical Phased Array probes have frequencies between 1MHz and 20MHz and the number of wafers is 10
...
to 128. M2 Electronics offers customers conventionally ultrasound probes and the ability to provide high-precision Phased Array Ultrasound Probes of up to 256 wafers. We can also customize the probe for our customers to meet the specific application requirements of the user.
>

Ultrasonic Squirter/Gantry System

TecScan’s NDT Ultrasonic Gantry Systems are industrial Squirter scanners designed for non-destru
...
ctive quality testing and raster scanning of large structures and parts. The Gantry Systems are usually composed of an Industrial Mechanical Scanner and a Control Room with System Workstation.
>

On-Demand Webinar: Advanced Ultrasonic Techniques for HTHA Detection & Assessment

In the Oil and Gas industry, High Temperature Hydrogen Attack (HTHA) can occur in steels operating a
...
t high temperature, usually above 205°C (400°F). Early stages of HTHA are very difficult to detect. Join us for a webinar presented by Guy Maes, Zetec Sales Engineer Director. The webinar will demonstrate how advanced ultrasonic techniques are deployed for efficient and reliable HTHA inspections.
>

Share...
We use technical and analytics cookies to ensure that we will give you the best experience of our website - More Info
Accept
top
this is debug window