STATISTICAL METHODS FOR QUALITY ASSURANCE Basics ...

1 This is page iPrinter: Opaque thisSTATISTICALMETHODS FORQUALITYASSURANCE: Basics , measurement , control , Capability, andImprovementStephen B. Vardeman and J. Marcus JobeSeptember 27, 2007iiThis is page iiiPrinter: Opaque thisContentsPrefacev1 The Nature of QUALITY and the Role of Statistics .. Modern QUALITY Philosophy and Business Practice Improvement Strate-gies .. Modern QUALITY Philosophy and a Six-Step Process-OrientedQuality ASSURANCE Cycle .. The Modern Business Environment and General Business ProcessImprovement .. Some Caveats .. Logical Process Identi cation and Analysis .. Elementary Principles of QUALITY ASSURANCE Data Collection .. Simple STATISTICAL Graphics and QUALITY ASSURANCE .. Chapter Summary .. Chapter 1 Exercises ..252 Statistics and Basic Concepts in Metrology and Probability Modeling of Measure-ment.

2 Elementary One- and Two-Sample STATISTICAL METHODS and Measure-ment .. One-Sample METHODS and measurement Error .. Two-Sample METHODS and measurement Error .. Some Intermediate STATISTICAL METHODS and measurement .. A Simple Method for Separating Process and MeasurementVariation .. One-Way Random Effects Models and Associated Inference . Gauge R&R Studies .. Two-Way Random Effects Models and Gauge R&R Studies . Range-Based Estimation .. ANOVA-Based Estimation .. Simple Linear Regression and Calibration Studies .. measurement Precision and the Ability to Detect a Change or R&R Considerations for Go/No-Go Inspection .. Some Simple Probability Modeling .. Simple R&R Point Estimates for 0/1 Contexts .. Application of Inference METHODS for the Difference in TwoBinomial "p's".

3 Chapter Summary .. Chapter 2 Exercises ..973 Process Generalities About Shewhart control Charting .. Shewhart Charts for Measurements/"Variables Data" .. Charts for Process Location .. Charts for Process Spread .. What ifn= 1? .. Shewhart Charts for Counts/"Attributes Data" .. Charts for Fraction Nonconforming .. Charts for Mean Nonconformities per Unit .. Patterns on Shewhart Charts and Special Alarm Rules .. The Average Run Length Concept .. STATISTICAL Process Monitoring and Engineering control .. Discrete Time PID control .. Comparisons and Contrasts .. Chapter Summary .. Chapter 3 Exercises ..174A The First Appendix205 Index206 This is page vPrinter: Opaque thisPrefaceThis is the preface. More here is page 1 Printer: Opaque thisCHAPTER1 IntroductionThis opening chapter rst introduces the subject of QUALITY ASSURANCE and the relation-ship between it and the subject of statistics in Section Then Section providescontext for the material of this book.

4 Standard emphases in modern QUALITY assuranceare introduced and a six-step process-oriented QUALITY ASSURANCE cycle is put forward asa framework for approaching projects in this eld. Some connections between modernquality ASSURANCE and popular business process improvement programs are discussednext. Some of the simplest QUALITY ASSURANCE tools are then introduced in through . There is a brief discussion of process mapping/analysis in ,.discussion of some simple principles of QUALITY ASSURANCE data collection followsin Section , and simple STATISTICAL graphics are considered in Section The Nature of QUALITY and the Role of StatisticsThis book's title raises at least two basic questions: "What is ` QUALITY '?" and "What do` STATISTICAL METHODS ' have to do with assuring it?"Consider rst the word " QUALITY .

5 " What does it mean to say that a particular good isa QUALITY product? And what does it mean to call a particular service a QUALITY service?In the case of manufactured goods (like automobiles and dishwashers), issues of relia-bility (the ability to function consistently and effectively across time), appropriatenessof con guration, and t and nish of parts come to mind. In the realm of services (liketelecommunications and transportation services) one thinks of consistency of avail-ability and performance, esthetics, and convenience. And in evaluating the " QUALITY "of both goods and services, there is an implicit understanding that these issues will be2 Chapter 1. Introductionbalanced against corresponding costs to determine overall "value." Here is a popularde nition of QUALITY that re ects some of these nition 1 Qualityin a good or service is tness for use.

6 That tness includes as-pects of both product design and conformance to the (ideal) of designhas to do with appropriateness; the choice and con guration offeatures that de ne what a good or service is supposed to be like and is supposed to many cases it is essentially a matter of matching product "species" to an arena of needs different things in a vehicle driven on the dirt roads of the Baja peninsulathan in one used on the German autobahn. Vehicle QUALITY of design has to do withproviding the "right" features at an appropriate price. With this understanding, thereis no necessary contradiction between thinking of both a Rolls Royce and a Toyotaeconomy car as QUALITY vehicles. Similarly, both a particular fast food outlet and aparticular four star restaurant might be thought of as QUALITY of conformancehas to do with living up to speci cations laid down inproduct design.

7 It is concerned with small variation from what is speci ed or inevitably makes goods and services undesirable. Mechanical devices whoseparts vary substantially from their ideal/design dimensions tend to be noisy, inef -cient, prone to breakdown, and dif cult to service. They simply don't work well. Inthe service sector, variation from what is promised/expected is the principal source ofcustomer dissatisfaction. A city bus system that runs on schedule every day that it issupposed to run can be seen as a QUALITY transportation system. One that fails to do socannot. And an otherwise elegant hotel that fails to ensure the spotless bathrooms itscustomers expect will soon be without those book is concerned primarily with tools for assuring QUALITY of is not because QUALITY of design is unimportant. Designing effective goods andservices is a highly creative and important activity.

8 But it is just not the primary topicof this what does the subject of statistics have to do with the ASSURANCE of QUALITY ofconformance? To answer this question, it is helpful to have clearly in mind a de nitionof nition 2 Statisticsis the study of how best data, or describe data, conclusions or inferences based on data,all in a framework that recognizes the reality and omnipresence of QUALITY of conformance has to do with small variation and one wishes to assureit, it will be necessary to measure, monitor, nd sources of, and seek ways to reducevariation. All of these require data (information on what is happening in a systemproducing a product) and therefore the tool of statistics. The intellectual frameworkChapter 1. Introduction 3of the subject of statistics, emphasizing as it does the concept of variation, makes it anatural for application in the world of QUALITY ASSURANCE .

9 We will see that both simpleand also somewhat more advanced METHODS of statistics have their uses in the quest toproduce QUALITY goods and Exercises1." QUALITY " and "statistics" are related. Brie y explain this relationship, using thede nitions of both is variation in manufactured parts undesirable? Why is variation undesir-able in a service industry? a product or service is designed appropriately, does that alone guarantee qual-ity? Why or why not? a product or service conforms to design speci cations, does that alone guar-antee QUALITY ? Why or why not? Modern QUALITY Philosophy and Business PracticeImprovement StrategiesThe global business environment is ercely competitive. No company can afford to"stand still" if it hopes to stay in business. Every healthy company has explicit strate-gies for constantly improving its business processes and the past several decades, there has been a blurring of distinctions between" QUALITY improvement" and "general business practice improvement.

10 " (Formerly, the rst of these was typically thought of as narrowly focused on characteristics of manu-factured goods.) So there is now much overlap in emphases, language, and methodolo-gies between the areas. The best strategies in both arenas must in the end boil down togood methodical/scienti c data-based problem this section we rst provide a discussion of some elements of modern qualityphilosophy and an intellectual framework around which we have organized the topicsof this book (and that can serve as a road map for approaching QUALITY improvementprojects). We then provide some additional discussion and critique of the modern gen-eral business environment and its better known process improvement Modern QUALITY Philosophy and a Six-Step Process-OrientedQuality ASSURANCE CycleModern QUALITY ASSURANCE METHODS and philosophy are focused not (primarily) on prod-ucts, but rather on theprocessesused to produce them.