Date: Wed, 17 Oct 2001 11:13:26 -0700
From: "Dr. Howard Castrup"
To: Greg Gogates
Subject: Traceability/Uncertainty
Mike, Steve, et al.,
In Traceability/Uncertainty RE08, Steve Ellison presents what I believe to be a cogent argument for a definition of traceability that is not tied to uncertainty propagation through a traceability chain. The following definition, quoted by Mehul Joshi in this thread, contains at least the elements of what should be preserved in discussing traceability:
Traceability: "The property of a result of measurement whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons."
Note that this definition does not include any mention of an accompanying uncertainty propagation analysis. Admittedly, requiring that parameters be calibrated by references that have themselves been calibrated by higher-level references in an unbroken chain traceable to a national or international standard or set of standards does not, in itself, control measurement uncertainty. However, such a requirement still has value. For one thing, the analysis of uncertainty propagation through a test and calibration support hierarchy, requires that the chain exist in the first place. Moreover, by leaving uncertainty propagation analysis out of the definition, we avoid several practical difficulties. These difficulties can be addressed by a separate uncertainty propagation analysis requirement.
In addition to traceability, other elements to be considered in uncertainty propagation analysis include the factors mentioned by Mike Ouellette in Traceability/Uncertainty RE03. These include location, environment, and time since calibration. The last factor is especially relevant to this discussion. As Mike and others have suggested or implied, the uncertainty of a calibrated parameter grows with time elapsed since calibration. With this in mind, it is important to note that the uncertainty statement that accompanies a calibrated parameter is nearly always just an estimate of the uncertainty of the calibration process -- not an estimate of the uncertainty in the reported parameter value at the time of use and under usage conditions.
I mention this as a lead-in to a comment that was made during an uncertainty analysis panel session at the 2001 International Dimensional Workshop. Responding to a question from the audience, one of the panelists, presumably in an effort to be consistent with the VIM, stated that, if the uncertainty during use is not the same as the stated uncertainty on the report of calibration, then the traceability of the calibrated parameter is lost.
Experience in the field of calibration interval analysis, replete with instances of observed changes in parameter value over time and/or reductions in in-tolerance probability as a function of time, argues that the uncertainty of a calibrated parameter begins to grow immediately following calibration and continues to grow during use. Given this, one might suspect that the reason ISO/IEC 17025 discourages projections of uncertainty over time on cal certs is that acknowledging the existence of uncertainty growth throws the VIM definition of traceability into controversy.
To see where this supposition derives from, I return to a comment made by Mike Ouellette: "... if the reported measurement is not accompanied by a (meaningful) uncertainty in that measurement, then THERE IS NO TRACEABILITY as defined by the VIM." Since a report of the measurement process uncertainty of a calibrating entity is hardly a meaningful uncertainty estimate for a parameter under conditions of use and time since calibration, by recognizing uncertainty growth, we either invalidate the VIM definition or conclude that we never have traceability.
So, Steve's objection to letting definitions dictate debate is needed, as we are apparently dealing with an example of the tail wagging the dog. The easiest solution, it seems to me, is to keep the concept of an unbroken chain of calibrations leading to a national or international standard separate from, but necessary to, the concept of uncertainty propagation through the chain. The former provides "error" control by fostering agreement between standards and equipment parameters, while the latter provides a means of assessing the quality of this control and also the quality of measurements made under conditions of use.
Dr. Howard Castrup
President, Integrated Sciences Group
From: "Dr. Howard Castrup"
To: Greg Gogates
Subject: Traceability/Uncertainty
Mike, Steve, et al.,
In Traceability/Uncertainty RE08, Steve Ellison presents what I believe to be a cogent argument for a definition of traceability that is not tied to uncertainty propagation through a traceability chain. The following definition, quoted by Mehul Joshi in this thread, contains at least the elements of what should be preserved in discussing traceability:
Traceability: "The property of a result of measurement whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons."
Note that this definition does not include any mention of an accompanying uncertainty propagation analysis. Admittedly, requiring that parameters be calibrated by references that have themselves been calibrated by higher-level references in an unbroken chain traceable to a national or international standard or set of standards does not, in itself, control measurement uncertainty. However, such a requirement still has value. For one thing, the analysis of uncertainty propagation through a test and calibration support hierarchy, requires that the chain exist in the first place. Moreover, by leaving uncertainty propagation analysis out of the definition, we avoid several practical difficulties. These difficulties can be addressed by a separate uncertainty propagation analysis requirement.
In addition to traceability, other elements to be considered in uncertainty propagation analysis include the factors mentioned by Mike Ouellette in Traceability/Uncertainty RE03. These include location, environment, and time since calibration. The last factor is especially relevant to this discussion. As Mike and others have suggested or implied, the uncertainty of a calibrated parameter grows with time elapsed since calibration. With this in mind, it is important to note that the uncertainty statement that accompanies a calibrated parameter is nearly always just an estimate of the uncertainty of the calibration process -- not an estimate of the uncertainty in the reported parameter value at the time of use and under usage conditions.
I mention this as a lead-in to a comment that was made during an uncertainty analysis panel session at the 2001 International Dimensional Workshop. Responding to a question from the audience, one of the panelists, presumably in an effort to be consistent with the VIM, stated that, if the uncertainty during use is not the same as the stated uncertainty on the report of calibration, then the traceability of the calibrated parameter is lost.
Experience in the field of calibration interval analysis, replete with instances of observed changes in parameter value over time and/or reductions in in-tolerance probability as a function of time, argues that the uncertainty of a calibrated parameter begins to grow immediately following calibration and continues to grow during use. Given this, one might suspect that the reason ISO/IEC 17025 discourages projections of uncertainty over time on cal certs is that acknowledging the existence of uncertainty growth throws the VIM definition of traceability into controversy.
To see where this supposition derives from, I return to a comment made by Mike Ouellette: "... if the reported measurement is not accompanied by a (meaningful) uncertainty in that measurement, then THERE IS NO TRACEABILITY as defined by the VIM." Since a report of the measurement process uncertainty of a calibrating entity is hardly a meaningful uncertainty estimate for a parameter under conditions of use and time since calibration, by recognizing uncertainty growth, we either invalidate the VIM definition or conclude that we never have traceability.
So, Steve's objection to letting definitions dictate debate is needed, as we are apparently dealing with an example of the tail wagging the dog. The easiest solution, it seems to me, is to keep the concept of an unbroken chain of calibrations leading to a national or international standard separate from, but necessary to, the concept of uncertainty propagation through the chain. The former provides "error" control by fostering agreement between standards and equipment parameters, while the latter provides a means of assessing the quality of this control and also the quality of measurements made under conditions of use.
Dr. Howard Castrup
President, Integrated Sciences Group