Quality Progress - July 2013 - 52

StatiSticS Roundtable BY LYnne B. hare Wanderlust and Memory How the EWMA model improves on Shewhart charts by accounting for process memory PoP quiz. Question one: How do you specifications. At best, it will operate within ed. If data are autocorrelated, their standard derive Shewhart (X-bar and R, or X-bar and its inherent, intrinsic capability. deviation is not the same as that of indepen- S) control chart limits? A. Control chart limits are the same as specification limits. But more than likely, there will be ran- dent samples taken from the same process. dom shocks and interventions of assignable And when you make decisions on the basis cause variation that disturb the process and of control charts, you are—knowingly or cause the end-product not—assuming independent samples. More moved toward the target variation to exceed on that later. from the specifications capability. Specifications limits to allow for sam- are, or should be, based of the above.” Control chart limits should pling variation. on product perfor- be based on the estimate of the capability C. Run the process and wait mance from an end-user standard deviation derived from a carefully until it achieves stability. perspective, not process conducted process capability study.1 Take samples of size four performance from a or five—or whatever is manufacturing perspec- Question two convenient—every few tive. All, or almost all, You get partial credit if you answered either B. Control chart limits are So the answer to question one is “none minutes. Calculate the average range of product should be within specification A or B. It is not that these reasons were the sets of these periodic samples and limits. part of Walter Shewhart’s intent in develop- multiply it by A2 from the control chart If you answered C, you may have been ing control charts, but it is true that many tables. Add the result to and subtract it influenced by some texts on statistical pro- organizations rely on control chart data as from the target to derive the limits. cess control or some training that recom- a legal record and, when needed, to show mends 10 easy steps to process perfection regulatory officials that they are on the ball. D. Take 30 consecutive samples. Calculate their standard deviation. Divide it by the that you saw on commercials listing 800 square root of the usual sample size. Mul- numbers on late-night TV. tiply that by three, and add and subtract the result from the target. E. None of the above. One clue to C being the wrong answer is Grudgingly, I’ll also give partial credit for C, but only because triggering process adjustment has become a common use for that if you wait until the process achieves control charts. “Wait a minute,” you say. stability, you could wait forever. Besides, if “I thought that’s what control charts are you actually waited for the achievement of for.” Not so. Shewhart’s original intent was Shewhart control chart? stability before you took data, how would you answer D: to detect the entry of assignable A. To please regulatory officials. ever know the process is stable? And how do cause variation in the process. If you want B. To maintain a process record for legal you measure stability in the first place? to signal the need for process adjustment, Question two: What is the purpose of a purposes. Answer D has two major flaws. One is C. To tell when to adjust the process. that those 30 consecutive samples may or D. To identify sources of assignable cause may not be representative of the process variation. short-term variation. Would you get the there are better devices than Shewhart control charts. The Shewhart model is: y = f(x) + ε, same result if you took a different set of meaning that some observed process value, 30 samples? Another flaw is that samples y, is a function of process settings, x, plus taken adjacently from most processes are noise, ε. The process mean is fixed, and Question one highly correlated. That is, when the value of the variation is experienced around that If you answered A or B, I’m sorry, but you one sample is high (low), it is likely that its fixed mean. This is a useful way to look got it wrong. Your process doesn’t know neighbor sample is also high (low). at processes, and as the late, amazingly E. None of the above. That’s it. Only two questions. about specifications; it doesn’t care about 52 QP • www.qualityprogress.com The results are considered autocorrelat- great George E.P. Box said, “All models 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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