Published on February 4, 2009
ANOVA Gauge R&R ANOVA Gauge R&R (or ANOVA Gauge Repeatability & Reproducibility) is a Measurement Systems Analysis technique which uses Analysis of Variance (ANOVA) random effects model to assess a measurement system. The evaluation of a measurement system is not limited to gauges (or gages) but to all types of measuring instruments and test methods. Contents 1 Purpose • 2 How to perform a Gauge R&R • 3 Common Misconceptions about GRR • 4 External References • 5 See also • Purpose ANOVA Gauge R&R measures the amount of variability induced in measurements that comes from the measurement system itself and compares it to the total variability observed to determine the viability of the measurement system. There are several components affecting a measurement system including: Measuring instruments, the gauge or instrument itself and all mounting blocks, • supports, fixtures, load cells etc. The machine ease of use, sloppiness among mating parts, quot;zeroquot; blocks are examples of sources of variation in the measurement system; Operators (people), the ability and/or discipline of a person to follow the written or • verbal instructions. Test methods, how to setup your devices, how to fixture your parts, how to record the • data, etc. Specification, the measurement is reported against a specification or a reference value. • The range of the specification does not affect the measurement, but is an important factor affecting the viability of the measurement system. Parts (what is being measured), some items are easier to measure than others. A • measurement system may be good for measuring steel block length but not for measuring rubber pieces. There are two important aspects on a Gauge R&R: Repeatability, the ability of the device to provide consistent results. It is a measure of • the variability induced by the system if the same operator measured the same part using the same device repeatedly.
Reproducibility, the variability induced by the operators. It is the variation induced • when different operators (or different laboratories) measure the same part. It is important to understand the difference between Accuracy and precision in order to understand the purpose of Gauge R&R. Gauge R&R only address how precise a measurement system is. Anova Gauge R&R is an important tool within the Six Sigma methodology, and is also a requirement for Production Part Approval Process (PPAP) documentation. How to perform a Gauge R&R The Gauge R&R is performed by measuring parts using the established measurement system. The goal is to capture as many sources of measurement variation as possible, so they can all be assessed and addressed. Please note that the purpose is not to quot;passquot;. A small variation reported on a GRR may be because an important source of error was missed during the study. In order to capture reproducibility errors, multiple operators are needed. Some (ASTM E691 Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method) call for at least ten operators (or laboratories) but others use only 2 or 3 to measure the same parts. In order to capture repeatability errors, the same part is usually measured several times per operator. In order to capture interactions of operators with parts (e.g. one part may be more difficult to measure than other), usually between 5 and 10 parts are measured. There is not a universal criteria for minimum requirements for the GRR matrix, being up to the Quality Engineer to assess risks depending on how critical the measurement is and how costly they are. The 10x2x2 (10 parts, 2 operators, 2 repetitions) is common, although it has very few degrees of freedom for the operator component. It is used as part of the Six Sigma DMAIC process for any variation project. it is imperative to use production parts, or parts that are similar to production parts (such as when performing pre-production evaluations). Common Misconceptions about GRR Need only one GRR per family of gauges. It is usual to say quot;There is an acceptable • GRR for this caliperquot;. This statement is false, as a GRR is for the measurement system, which includes the part, specification, operator and method. As an example, measuring a steel block with a caliper may be achieved with a good precision, but the same caliper may not be suitable to measure soft rubber parts that may deform while it is being measured. The GRR will not pass using parts, so it has to be done with standard weights and • blocks. The GRR done in this way will assess the precision while measuring standard weights. The device might not be suitable to measure that specific type of parts. If the
part quot;changesquot; while being measured, this has to be counted as a measurement system error. Need to report on PPAP documentation GRR results for everything that is • measured. This is not necessarily a requirement. The Quality Engineer usually makes an educated assessment. If the characteristic is critical to safety, a valid GRR is required. Instead, if there is enough understanding that some particular part is easy to measure with acceptable precision, a formal GRR is not required. Customers may ask for additional GRRs during PPAP reviews. Knowing that a GRR is not good and still uses the measurement system does not make sense. This is like using bent calipers to get measurements, you get a number but it does not mean anything. Performing a GRR is very expensive. In order to perform a GRR usually a number of • parts (sometimes between 5 to 10) is required to be measured by at least 3 operators (some suggest ten or more) 2 to 3 times. So the measurement costs are the ones associated with those additional measurements. For simple devices this may not be very costly, and the results is a known measurement error that can be used to assess all measurements subsequent to that. The costs can be higher for destructive testing. GRRs must be within 10% to pass. There are AIAG guidelines for GRR errors • relative to the specification, and what to report on a PPAP process. The final call is between the supplier and customer, and it is a function of the critically of the characteristic and the assessed measurement error. GRR is a tool that helps making this assessment, but it does not gives you the answer. Gauge Repeatability and Reproducibility (GR&R) • Gauge Repeatability and Reproducibility, or GR&R, is a measure of the capability of a gauge or gage to obtain the same measurement reading every time the measurement process is undertaken for the same characteristic or parameter. In other words, GR&R indicates the consistency and stability of measuring equipment. The ability of a measuring device to provide consistent measurement data is important in the control of any process. • Mathematically, GR&R is actually a measure of the variation of a gage's measurement, and not of its stability. An engineer must therefore strive to minimize the GR&R numbers of his or her measuring equipment, since a high GR&R number indicates instability and is thus undesirable. • As its name implies, GR&R (or simply 'R&R') has two major components, namely, repeatability and reproducibility. Repeatability is the ability of the same gage to give consistent measurement readings no matter how many times the same operator of the gage repeats the measurement process. Reproducibility, on the other hand, is the ability of the same gage to give consistent measurement readings regardless of who performs the measurements. The evaluation of a gage's reproducibility, therefore, requires measurement readings to be acquired by different operators under the same conditions. • Of course, in the real world, there are no existing gages or measuring devices that give exactly the same measurement readings all the time for the same parameter. There are five (5) major elements of a measurement system, all of which contribute to the variability of a measurement process: 1) the standard; 2) the workpiece; 3) the instrument; 4) the people; and 5) the environment.
All of these factors affect the measurement reading acquired during each measurement cycle, • although to varying degrees. Measurement errors, therefore, can only be minimized if the errors or variations contributed individually by each of these factors can also be minimized. Still, the gage is at the center of any measurement process, so its proper design and usage must be ensured to optimize its repeatability and reproducibility. • There are various ways by which the R&R of an instrument may be assessed, one of which is outlined below. This method, which is based on the method recommended by the Automotive Industry Action Group (AIAG), first computes for variations due to the measuring equipment and its operators. The over-all GR&R is then computed from these component variations. • Equipment Variation, or EV, represents the repeatability of the measurement process. It is calculated from measurement data obtained by the same operator from several cycles of measurements, or trials, using the same equipment. Appraiser Variation or AV represents the reproducibility of the measurement process. It is calculated from measurement data obtained by different operators or appraisers using the same equipment under the same conditions. The R&R is just the combined effect of EV and AV. • It must be noted that measurement variations are caused not just by EV and AV, but by Part Variation as well, or PV. PV represents the effect of the variation of parts being measured on the measurement process, and is calculated from measurement data obtained from several parts. • Thus, the Total Variation (TV), or the over-all variation exhibited by the measurement system, consists of the effects of both R&R and PV. TV is equal to the square root of the sum of (R&R)2 and (PV)2 square, i.e., • TV = √ (R&R)2 + PV2. • In a GR&R report, the final results are often expressed as %EV, %AV, %R&R, and %PV, which are simply the ratios of EV, AV, R&R, and PV to TV expressed in %. Thus, %EV=(EV/TV)x100%; %AV=(AV/TV)x100%; %R&R=(R&R/TV)x100%; and %PV=(PV/TV)x100%. The gage is good if its %R&R is less than 10%. A %R&R between 10% to 30% may also be acceptable, depending on what it would take to improve the R&R. A %R&R of more than 30%, however, should prompt the process owner to investigate how the R&R of the gage can be further improved. Repeatability: Repeatability is the variation in measurements observed when one operator repeatedly • measures the same characteristic in the same place on the same part with the same gauge: (i.e. variation in measurements under identical conditions). (See Reproducibility) A limit within which agreement may be expected 95% of the time between two test • results obtained in essentially the same conditions and from the same homogeneous sample of material. Repeatability is the inherent variation within the measurement instrument (gauge) and • is represented by repeatability, which is the standard deviation of the measurement instrument. Reproducibility: Reproducibility is the variation in average measurements due to factors other than the • machine variation. These factors include, but are not limited to, operators, different gauges, temperature, • humidity, and part fixturing technique. (See Repeatability) Reproducibility is the variation due to the measurement system. • R & R studies can partition reproducibility into these component parts. •
The partitioning can be used to decide where the problem areas are and improve the • measuring system.
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