Ch04 spc


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Ch04 spc

  1. 1. Quality Improvement Statistical Process Control
  2. 2. Outline  Pareto Diagram  Cause-Effect Diagram  Check Sheets  Process Flow Diagram  Scatter Diagram  Histogram  Control Charts 2
  3. 3. Learning Objectives When you have completed this chapter you should be able to:  Construct a Pareto diagram.  Explain how to construct a cause and effect diagram.  Explain how to construct a check sheet.  Construct a process flow chart. 3
  4. 4. Statistical Process Control A methodology for monitoring a process to identify special causes of variation and signal the need to take corrective action when appropriate 4
  5. 5. The Pareto Principle Vilfredo Pareto was an economist who is credited with establishing what is now widely known as the Pareto Principle or 80/20 rule. When he discovered the principle, it established that 80% of the land in Italy was owned by 20% of the population. Later, he discovered that the pareto principle was valid in other parts of his life, such as gardening: 80% of his garden peas were produced by 20% of the peapods. 5
  6. 6. The Pareto Principle Some Sample 80/20 Rule Applications  80% of process defects arise from 20% of the process issues.  20% of your sales force produces 80% of your company revenues.  80% of delays in schedule arise from 20% of the possible causes of the delays.  80% of customer complaints arise from 20% of your products or services. (The above examples are rough estimates.) 6
  7. 7. The Pareto Diagram  Graph that ranks data classifications in descending order from left to right  Pareto diagrams are used to identify the most important problems  Advantage: Provide a visual impact of those vital few characteristics that need attention  Resources are then directed to take the necessary corrective action 7
  8. 8. The Pareto Diagram  Helps a team focus on causes that have the greatest impact  Displays the relative importance of problems in a simple visual format  Helps prevent “shifting the problem” where the solution removes some causes but worsens others 8
  9. 9. Constructing a Pareto Diagram Steps: 1. Determine the method of classifying the data: by problem, cause, type of nonconformity, etc 2. Decide if dollars (best), weighted frequency, or frequency is to be used to rank the characteristics 3. Collect data for an appropriate time interval 9
  10. 10. Constructing a Pareto Diagram Steps cont’d: 4. Summarize the data and rank order categories from largest to smallest 5. Compute the cumulative percentage if it is to be used 6. Construct the diagram and find the vital few 10
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  12. 12. Cause-and-Effect Diagram  It was developed by Dr. Kaoru Ishikawa in 1943  Picture composed of lines and symbols designed to represent a meaningful relationship between an effect and its causes  Effect (characteristics that need improvement) on the right and causes on the left 12
  13. 13. Cause-and-Effect Diagram Figure 4.3 Cause-and-Effect Diagram 13
  14. 14. Cause-and-Effect Diagram  Enables a team to focus on the content of a problem, not on the history of the problem or differing personal interests of team members  Creates a snapshot of collective knowledge and consensus of a team; builds support for solutions  Focuses the team on causes, not symptoms  Used to investigate either a “bad” effect and to take action to correct the causes or a “good” effect and to learn those causes responsible 14
  15. 15. Cause-and-Effect Diagram Steps in the construction of a Cause-and-Effect Diagram: 1. Identify the effect or quality problem 2. Determine the major causes 3. Determine all the minor causes. Request a brainstorming session 4. Once the diagram is complete, evaluate it to determine the most likely causes 5. Develop solutions 15
  16. 16. Cause-and-Effect Diagram Advantages: 1. Analyzing actual conditions for the purpose of product or service quality improvement 2. Elimination of conditions causing nonconforming product or service and customer complaints 3. Standardization of existing and proposed operations 4. Education and training in decision-making 16
  17. 17. Cause-and-Effect Diagram Types of Diagrams: 1. The dispersion-analysis type.  Each major branch is filled in completely before starting work on any of the other branches. The objective is to analyze the causes of dispersion or variability 17
  18. 18. Cause-and-Effect Diagram Types of Diagrams: 2. The process-analysis type.  In order to construct it, it is necessary to write each step of the production process. The advantage of this type of diagram is the ease of construction and its simplicity, since it follows the production sequence 18
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  20. 20. Check Sheets  The main purpose is to ensure that the data are collected carefully and accurately by operating personel for process control and problem solving  The form of the check sheet is individualized for each situation and is designed by the project team  Check sheets are designed to show location 20
  21. 21. Check Sheets  Creates easy-to-understand data  Builds, with each observation, a clearer picture of the facts  Forces agreement on the definition of each condition or event of interest  Patterns in the data become obvious quickly  Creativity plays a major role in the design of a check sheet 21
  22. 22. Check Sheets Figure 3-5 Check Sheet for paint non-conformities 27
  23. 23. Figure 3-6 Check Sheet for swimming pool 28
  24. 24. Check Sheets Plastic Mold
  25. 25. Process Flow Diagram  It is a schematic diagram that shows the flow of the product or service as it moves through the various processing stations or operations  Makes it easy to visualize the entire system, identify potential trouble spots, and locate activities  Compares and contrasts actual versus ideal flow of a process 25
  26. 26. Process Flow Diagram 26
  27. 27. Process Flow Diagram  Serves as a training tool  Uses standardized symbols  Shows unexpected complexity, problem areas, redundancy, unnecessary loops, and where simplification may be possible 27
  28. 28. Process Flow Diagram  Allows a team to reach agreement on process steps and identify activities that may impact performance  Improvements to the process can be accomplished by eliminating steps, combining steps, or making frequently occurring steps more efficient 28
  29. 29. Scatter Diagram The simplest way to determine if a cause and-effect relationship exists between two variables. Details are in Chapter 5. 29
  30. 30. Histogram Graphically shows the process capability and, if desired, the relationship to the specifications and the nominal Figure 3-11 Histogram for Hole Location 30
  31. 31. Histogram  Displays large amounts of data that are difficult to interpret in tabular form  Shows centering, variation, and shape  Illustrates the underlying distribution of the data  Provides useful information for predicting future performance  Helps to answer the question “Is the process capable of meeting requirements? 31
  32. 32. Control Charts  Focuses attention on detecting and monitoring process variation over time  Distinguishes special from common causes of variation  Serves as a tool for on-going control  Provides a common language for discussion process performance  Details in Chapters 6,7, & 9 32
  33. 33. Control Chart 33