Significance and Use
5.1 The POD analysis method described herein is based on well-known and well-established statistical methods. It shall be used to quantify the demonstrated POD for a specific set of examination parameters and known range of discontinuity sizes under the following conditions.
5.1.1 The initial response from a nondestructive evaluation inspection system is measurable and can be classified as a continuous variable.
5.1.2 Discontinuity size is the predictor variable and can be accurately quantified.
5.1.3 The relationship between discontinuity size (a) and measured signal response (â) exists and is best described by a linear regression model with an error structure that is normally distributed with mean zero and constant variance, σ2. (Note that in linear regression, “linear” means linear with respect to the model coefficients. Though a quadratic model does not have a linear shape when plotted, for example, it is classified as a linear model in regression analysis since it is linear with respect to the model coefficients.)
5.2 This practice does not limit the use of a linear regression model with more than one predictor variable or other statistical models if justified as more appropriate for the â versus a data.
5.3 This practice is not appropriate for data resulting from a POD examination on nondestructive evaluation systems that generate an initial response that is binary in nature (for example, hit/miss). Practice is appropriate for systems that generate a hit/miss-type response (for example, fluorescent penetrant).
5.4 Prior to performing the analysis, it is assumed that the discontinuity of interest is clearly defined; the number and distribution of induced discontinuity sizes in the POD specimen set is known and well documented; the POD examination administration procedure (including data collection method) is well designed, well defined, under control, and unbiased (see for more detail); the initial inspection system response is measurable and continuous in nature; the inspection system is calibrated; and the measurement error has been evaluated and deemed acceptable. The analysis results are only valid if the â versus a data are accurate and precise and the linear model adequately represents the â versus a data.
5.5 The POD analysis method described herein is consistent with the analysis method for â versus a data described in MIL-HDBK-1823A and is included in several widely utilized POD software packages to perform a POD analysis on â versus a data. It is also found in statistical software packages that have linear regression analysis capability. This practice requires that the analyst has access to either POD software or other software with linear regression analysis capability.
1.1 This practice defines the procedure for performing a statistical analysis on Nondestructive Testing (NDT) â versus a data to determine the demonstrated probability of detection (POD) for a specific set of examination parameters. Topics covered include the standard â versus a regression methodology, POD curve formulation, validation techniques, and correct interpretation of results.
1.2 Units—The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.