# Quality Progress - November 2014 - 8

EXPERTANSWE Evaluating repeatability als under fixed and defined Q: From a statistical point of view, what is conditions of measurement" the better way to evaluate the repeatability (Figure 2).2 of an instrument: to repeat the measure- On the other hand, unifor- ment in each calibration point three times, mity is "the difference in varia- or to take only one measurement in all the tion throughout the operating calibration points except one, in which five range of the gage" or "the ho- measurements are taken? mogeneity (sameness) of the Test of variances for uniformity assessment / FIGURE 3 Pablo Alra cision as "the net effect of dis- Location 1 Bartlett's test Location 2 P-value 0.125 repeatability over size."3 Pre- Campana, Buenos Aires, Argentina Test for equal variances: locations 1, 2, 3, 4, 5 crimination, sensitivity and reA: You must understand the definition peatability over the operating of repeatability before answering this range (size, range and time) of question. The automotive Industry Action Location 3 Location 4 the measurement system."4 Group's Measurement Systems Analysis Location 5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 95% Bonferroni conﬁdence intervals for standard deviations Your question combines (MSA) Manual explains repeatability as repeatability and uniformity. "the variation in measurements obtained You cannot calculate repeatability with just complex measurement system, however, with one measurement instrument when one measurement. Taking more measure- inconsistency may be significant. used several times by one appraiser while ments reduces the error in the repeatability measuring the identical characteristic on measurement. If you are interested in estimated repeatability standard devia- the same part" (Figure 1).1 estimating uniformity, you must take more tion.5 The d2 constant is selected based on measurements at select intervals in your the number of replications (trials). As you operating range, calculate repeatability make more replications, the repeatability The manual also explains that "this is the variation in successive (short-term) tri- The MSA manual provides a formula for over the operating range and / FIGURE 1 standard deviation improves. Four replica- verify if there is homogeneity tions improve the repeatability by half. (Figure 3 and Table 1). Repeatability trials It will take an additional six replications The MSA manual points to improve it by one-third. I would say out that for simple measuring Measurement and distribution of replicated measurements / FIGURE 2 _ X making four replications is cost effective equipment, the repeatability (Figure 4). and uniformity are expected One additional benefit with measure- to be consistent. For a more ment points at multiple locations (using a Measurement trials across multiple locations / TABLE 1 Location 1 Frequency Trial 1 Location 3 Location 4 Location 5 9.99 9.956 10.001 9.988 Trial 2 σ SGage 5.15 SGage QP * www.qualityprogress.com 10.049 9.963 9.975 9.968 10.001 10.022 10.059 9.969 10.031 Trial 4 0.5% 0.5% 10.03 Trial 3 Width 8 Location 2 9.979 9.998 9.997 9.91 10.039 10.011 Average 10.002 10.015 9.972 9.996 10.000 Range 0.0510 0.0590 0.1490 0.0700 0.0630 Standard deviation 0.0211 0.0268 0.0626 0.0319 0.0274 http://www.qualityprogress.com

Up Front
LogOn
Keeping Current
Finding Harmony
Multipronged Approach
Bank on It
The System Is the Solution
Innovation Imperative
Quality in the First Person
Career Corner
Statistics Roundtable
Standards Outlook
QP Toolbox
QP Reviews
One Good Idea

# Quality Progress - November 2014

http://www.nxtbook.com/naylor/ASQM/ASQM0719
http://www.nxtbook.com/naylor/ASQM/ASQM0619
http://www.nxtbook.com/naylor/ASQM/ASQM0519
http://www.nxtbook.com/naylor/ASQM/ASQM0419
http://www.nxtbook.com/naylor/ASQM/ASQM0319
http://www.nxtbook.com/naylor/ASQM/ASQM0219
http://www.nxtbook.com/naylor/ASQM/ASQM0119
http://www.nxtbook.com/naylor/ASQM/ASQM1218
http://www.nxtbook.com/naylor/ASQM/ASQM1118
http://www.nxtbook.com/naylor/ASQM/ASQM1018
http://www.nxtbook.com/naylor/ASQM/ASQM0918
http://www.nxtbook.com/naylor/ASQM/ASQM0818
http://www.nxtbook.com/naylor/ASQM/ASQM0718
http://www.nxtbook.com/naylor/ASQM/ASQM0618
http://www.nxtbook.com/naylor/ASQM/ASQM0518
http://www.nxtbook.com/naylor/ASQM/ASQM0418
http://www.nxtbook.com/naylor/ASQM/ASQM0318
http://www.nxtbook.com/naylor/ASQM/ASQM0218
http://www.nxtbook.com/naylor/ASQM/ASQM0118
http://www.nxtbook.com/naylor/ASQM/ASQM1217
http://www.nxtbook.com/naylor/ASQM/ASQM1117
http://www.nxtbook.com/naylor/ASQM/ASQM1017
http://www.nxtbook.com/naylor/ASQM/ASQM0917
http://www.nxtbook.com/naylor/ASQM/ASQM0817
http://www.nxtbook.com/naylor/ASQM/ASQM0717
http://www.nxtbook.com/naylor/ASQM/ASQM0617
http://www.nxtbook.com/naylor/ASQM/ASQM0517
http://www.nxtbook.com/naylor/ASQM/ASQM0417
http://www.nxtbook.com/naylor/ASQM/ASQC12518
http://www.nxtbook.com/naylor/ASQM/ASQM0317
http://www.nxtbook.com/naylor/ASQM/ASQM0217
http://www.nxtbook.com/naylor/ASQM/ASQM0117
http://www.nxtbook.com/naylor/ASQM/ASQM1216
http://www.nxtbook.com/naylor/ASQM/ASQM1116
http://www.nxtbook.com/naylor/ASQM/ASQM1016
http://www.nxtbook.com/naylor/ASQM/ASAC0016
http://www.nxtbook.com/naylor/ASQM/ASQM0916
http://www.nxtbook.com/naylor/ASQM/ASQA0016
http://www.nxtbook.com/naylor/ASQM/ASQM0816
http://www.nxtbook.com/naylor/ASQM/ASQM0716
http://www.nxtbook.com/naylor/ASQM/ASQM0616
http://www.nxtbook.com/naylor/ASQM/ASQM0516
http://www.nxtbook.com/naylor/ASQM/ASQM0416
http://www.nxtbook.com/naylor/ASQM/ASQM0316
http://www.nxtbook.com/naylor/ASQM/ASQM0216
http://www.nxtbook.com/naylor/ASQM/ASQM0116
http://www.nxtbook.com/naylor/ASQM/ASQM1215
http://www.nxtbook.com/naylor/ASQM/ASQM1115
http://www.nxtbook.com/naylor/ASQM/ASQM1015
http://www.nxtbook.com/naylor/ASQM/ASQM0915
http://www.nxtbook.com/naylor/ASQM/ASQM0815
http://www.nxtbook.com/naylor/ASQM/ASQM0715
http://www.nxtbook.com/naylor/ASQM/ASQM0615
http://www.nxtbook.com/naylor/ASQM/ASQM0515
http://www.nxtbook.com/naylor/ASQM/ASQM0315
http://www.nxtbook.com/naylor/ASQM/ASQM0215
http://www.nxtbook.com/naylor/ASQM/ASQM0115
http://www.nxtbook.com/naylor/ASQM/ASQM1214
http://www.nxtbook.com/naylor/ASQM/ASQM1114
http://www.nxtbook.com/naylor/ASQM/ASQM1014
http://www.nxtbook.com/naylor/ASQM/ASQM0914
http://www.nxtbook.com/naylor/ASQM/ASQM0814
http://www.nxtbook.com/naylor/ASQM/ASQM0714
http://www.nxtbook.com/naylor/ASQM/ASQM0614
http://www.nxtbook.com/naylor/ASQM/ASQM0514
http://www.nxtbook.com/naylor/ASQM/ASQM0414
http://www.nxtbook.com/naylor/ASQM/ASQM0314
http://www.nxtbook.com/naylor/ASQM/ASQM0214
http://www.nxtbook.com/naylor/ASQM/ASQM0114
http://www.nxtbook.com/naylor/ASQM/ASQM1213
http://www.nxtbook.com/naylor/ASQM/ASQM1113
http://www.nxtbook.com/naylor/ASQM/ASQM1013
http://www.nxtbook.com/naylor/ASQM/ASQM0913
http://www.nxtbook.com/naylor/ASQM/ASQM0813
http://www.nxtbook.com/naylor/ASQM/ASQM0713
http://www.nxtbook.com/naylor/ASQM/ASQM0613
http://www.nxtbook.com/naylor/ASQM/ASQM0513
http://www.nxtbook.com/naylor/ASQM/ASQM0413
http://www.nxtbook.com/naylor/ASQM/ASQM0313
http://www.nxtbook.com/nxtbooks/naylor/ASQM0213
http://www.nxtbook.com/nxtbooks/naylor/ASQM0113
http://www.nxtbook.com/nxtbooks/naylor/ASQM1212
http://www.nxtbook.com/nxtbooks/naylor/ASQM1112
http://www.nxtbook.com/nxtbooks/naylor/ASQM1012
http://www.nxtbook.com/nxtbooks/naylor/ASQM0912
http://www.nxtbookMEDIA.com