Quality Progress - July 2013 - 8

ExPERTANsWE What’s acceptable? All this leads to the recommenda- Q: A handheld micrometer is to be used to tion to think separately about calibration perform measurement. The manufacturer and total measurement system varia- states that the micrometer has a resolution tion. Ensuring accuracy is always a good of 0.001 millimeters and an accuracy of thing, so calibrate the micrometer to the 0.001 millimeters. Given our situation, we highest reasonable criteria. Then, perform feel we’ll be lucky if our results of measure- measurement systems analysis (such as ment will be within 0.005 millimeters. This GR&R) studies to identify and reduce the is acceptable, as the product being mea- largest contributors to the total variation sured has a tolerance of +/-0.05 millime- in the particular application. To answer the ters. This gives a test accuracy ratio (TAR) of question directly, between the options, 10:1. We want to use this micrometer. Now the question is: When we calibrate Assuming we use 10:1, the implication for this example is that the reference stan- calibrating to 0.001 millimeters should be possible and is preferable. the micrometer in-house, should the ac- dard should have an accuracy of at least ceptable tolerance be that of the manufac- 0.0001 millimeters. In practice, then, typical turer, which is +/-0.001 millimeters, or ours, gauge blocks or rods with this accuracy which is +/-0.005 millimeters? We don’t can be used to adequately calibrate this Teleflex Inc. believe that +/-0.001 millimeters is realistic micrometer to the desired level. TAR can be Reading, PA for us. confused with resolution because another Scott A. Laman Senior manager, quality engineering and risk management Bob Kennedy rule of thumb is that it is desirable to be Developing quality indicators Sligo, Ireland able to detect differences in measurements Q: I am developing a project quality indica- as small as 1/10 of the total tolerance or tor for oil and gas projects. Can anyone process spread. guide or share some examples on what A: This answer contains various definitions and explanations of terms, and hopefully A separate issue is that the measure- should be included in this indicator? helps distinguish among different mea- ments of the desired parts may be able to surement concepts and types of variation. be obtained to only 0.005 millimeters accu- When the manufacturer states that the racy. Most of the variation in the 0.005 mil- micrometer has a resolution of 0.001 milli- limeters comes from typical gage repeat- A: The term “indicator” is used to mean meters, that means the smallest difference ability and reproducibility (GR&R) factors. observable measures that provide insights that can be measured is 0.001 millimeters. Repeatability is a measure of the ability of into a concept. The term “quality” is more When speaking about accuracy, the 0.001 the measuring system to obtain the same difficult to measure directly. However, for millimeters refers to the degree of agree- value when all factors are kept as constant a more precise definition, quality perfor- ment between the measured value and a as possible, and is often called equipment mance indicators can be divided into two true value, with the true value being that of variation. Repeatability is a measure of the types: outcome indicators and activities a reference standard. micrometer’s precision, and is independent indicators. Calibration verifies accuracy and subse- of the calibration status. Reproducibility is Ashwani Kumar Khare Abu Dhabi, United Arab Emirates Both outcome and activities indicators quent adjustment improves accuracy, but consist of two key components: resolution is not affected. TAR is the ratio tion, and is considered a measure of the 1. A definition that clearly states what is of the accuracy of the tool being calibrated ability of the measuring system to obtain (such as a micrometer) to the accuracy of the same value under varying conditions, the reference standard. Note that the TAR most commonly different operators at dif- is not a function of the product tolerance. A ferent times. Reproducibility is often called surement or how the indicator is being TAR of 10:1 or 4:1 is considered typical. 8 often a larger contributor to the total varia- appraiser variation. measured and is precise enough to QP • www.qualityprogress.com being measured in meaningful terms for the intended audience. 2. A metric that defines the unit of mea- http://www.qualityprogress.com

Table of Contents for the Digital Edition of Quality Progress - July 2013

Up Front
LogOn
Expert Answers
Keeping Current
Mr. Pareto Head
Feeling Connected
Take Charge
Full View
Reinventing Excellence
Innovation Imperative
Measure for Measure
Quality in the First Person
Standards Outlook
Career Corner
Statistics Roundtable
ASQ's Continuing Education and Professional Development Directory
QP Toolbox
QP Reviews
One Good Idea

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