6-1: Explain the difference between quality control and quality assurance.

Quality control is concerned with the quality of a product or service after it is produced or delivered. By contrast, quality assurance (QA) is a set of procedures to ensure a product or service’s quality before it is delivered. Various quality appraisal and quality control tools are covered in the chapter.

6-2: Apply the various tools for appraising the quality of products and processes.

The appraisal tools that managers and employees use include check sheets, histograms, Pareto charts, scatter diagrams, cause-and-effect diagrams, and process flowcharts. Check sheets are forms used to collect and record quality-related data. The sheets are one of the simplest quality control tools because they are easy to understand, require very little effort to design, and are fact-based. A histogram is a vertical bar chart that shows the frequency of occurrences of values. A Pareto chart is also a vertical bar chart with the bars arranged in decreasing height from left to right. The height of an individual bar represents how often a particular problem has occurred. A scatter diagram is a tool that can be used to visually determine whether two variables are related or correlated. A cause-and-effect diagram is a quality control tool used to identify, sort, and graphically display the potential causes of a quality problem. A process flowchart graphically displays the steps in a process and its flow. Before beginning any process quality improvement effort, it is a good idea to depict the process in its current state using a flowchart.

6-3: Apply the various tools for preventing defects in products and processes, including control charts, a process capability analysis, and how to calculate Six Sigma levels of quality.

The three basic statistical process control methods are: control charts, process capability analysis, and assessing Six Sigma–level quality. The chapter describes the difference between variable and attribute quality and the underlying theory behind control charts. Two control charts for monitoring variable quality and two control charts for attribute quality are discussed. Process capability analysis is used to determine whether the process is stable, centered, and the process output falls within specification limits. In a Six Sigma process, a defect is defined as anything that falls outside of customer specifications limits (USL and LSL). This means 99.9997% of the time the value of a quality characteristic should be between six standard deviations of the mean and the lower or upper specification limit. Thus, once the specification limits are established, the quality level of a process on the Six Sigma scale can be calculated by determining the distance between the mean and the specification limits in terms of the actual standard deviation of the process. 

6-4: Use quality design tools to improve product or process design.

The concept of robust design involves designing a product or process so that it is insensitive to factors such as different environmental conditions, machine wear and tear, variation resulting from differences in raw materials, and so forth. Three of the most important robust design tools are the Taguchi-loss function, parameter design process, and design of experiments (DOE). The costs associated with poor quality can be evaluated using the Taguchi-loss function. These costs of poor quality provide valuable information to quality managers to reduce variation using robust design methods. By using parameter design and design of experiment tools, product and process designers can establish product parameter settings that are robust to variations caused by uncontrollable factors. As a result, the designers can determine the optimal design of a product and the processes used to produce it so as to minimize variations.

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