tailieunhanh - Ebook Fundamentals of quality control and improvement: Part 2

(BQ) Part 2 book "Fundamentals of quality control and improvement" has contents: Control charts for attributes, process capability analysis, acceptance sampling plans for attributes and variables, reliability, experimental design and the taguchi method. | 8 CONTROL CHARTS FOR ATTRIBUTES 8-1 8-2 8-3 8-4 8-5 8-6 8-7 8-8 8-9 8-10 Introduction and chapter objectives Advantages and disadvantages of attribute charts Preliminary decisions Chart for proportion nonconforming: p-Chart Chart for number of nonconforming items: np-Chart Chart for the number of nonconformities: c-Chart Chart for number of nonconformities per unit: «-Chart Chart for demerits per unit: [/-Chart Charts for highly conforming processes Operating characteristic curves for attribute control charts Summary 8-1 INTRODUCTION AND CHAPTER OBJECTIVES In Chapter 7 we discussed statistical process control using control charts for variables. In this chapter we examine control charts for attributes. An attribute is a quality characteristic for which a numerical value is not specified. It is measured on anominal scale; that is, it does or does not meet certain guidelines, or it is categorized according to a scheme of labels. For instance, the taste of a certain dish is labeled as acceptable or unacceptable, or is categorized as exceptional, good, fair, or poor. Our objective is to present various types of control charts for attributes. A quality characteristic that does not meet certain prescribed standards (or specifications) is said to be a nonconformity (or defect). For example, if the length of steel bars is expected to be 50 ± cm, a length of cm is not acceptable. A product with one or more nonconformities, such that it is unable to meet the intended standards and is unable to function as required, is a nonconforming item (or defective). It is possible for a product to have several nonconformities without being classified as a nonconforming item. The different types of control charts considered in this chapter are grouped into three categories. The first category includes control charts that focus on proportion: the proportion of nonconforming items (p-chart) and the number of nonconforming items (np-chart). These two charts are based on binomial .