Seven Quality Control Tools

Seven Quality Control Tools

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Pareto analysis Flowcharts Check sheets Histograms Scatter diagrams Control charts Fishbone diagram

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Vilfredo Pareto was an Italian economist who noted that approximately 80% of wealth was owned by only 20% of the population. This was true in almost all the societies he studied.

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1. Pareto analysis  

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Choosing the Most Important Changes to Make Pareto analysis is a very simple technique that helps you to choose the most effective changes to make. It uses the Pareto principle - the idea that by doing 20% of work you can generate 80% of the advantage of doing the entire job. Pareto analysis is a formal technique for finding the changes that will give the biggest benefits. It is useful where many possible courses of action are competing for your attention.

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It is a statistical technique in decision making that is used for selection of a limited number of tasks that produce significant overall effect. It uses the Pareto principle - the idea that by doing 20% of work you can generate 80% of the advantage of doing the entire job. Or in terms of quality improvement, a large majority of problems (80%) are produced by a few key causes (20%). 5

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Pareto analysis is a formal technique useful where many possible courses of action are competing for your attention. Basically, it consists of estimating the benefit delivered by each action with subsequent selection of a number of the most effective actions that deliver the total benefit reasonably close to the maximum possible one.

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Write out a list of the changes you could make. If you have a long list, group it into related changes. Then score the items or groups. The scoring method you use depends on the sort of problem you are trying to solve.

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For example, if you are trying to improve profitability, you would score options on the basis of the profit each group might generate. If you are trying to improve customer satisfaction, you might score on the basis of the number of complaints eliminated by each change. 8

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The first change to tackle is the one that has the highest score. This one will give you the biggest benefit if you solve it. The options with the lowest scores will probably not even be worth bothering with solving these problems may cost you more than the solutions are worth.

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Case Study Implementation of Pareto analysis.

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A failing service center 

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Phones are only answered after many rings. Staff seem distracted and under pressure. Engineers do not appear to be well organized. They need second visits to bring extra parts. This means that customers have to take more holiday to be there a second time. 11

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They do not know what time they will arrive. This means that customers may have to be in all day for an engineer to visit. Staff members do not always seem to know what they are doing. Sometimes when staff members arrive, the customer finds that the problem could have been solved over the phone 12

Grouping & Scoring the problems.    

Lack of staff training: items 5 and 6: 51 complaints Too few staff: items 1, 2 and 4: 21 complaints Poor organization and preparation: item 3: 2 complaints By doing the Pareto analysis above, the manager can better see that the vast majority of problems (69%) can be solved by improving staff skills.

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Once this is done, it may be worth looking at increasing the number of staff members. Alternatively, as staff members become more able to solve problems over the phone, maybe the need for new staff members may decline. It looks as if comments on poor organization and preparation may be rare, and could be caused by problems beyond the manager's control. 14

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By carrying out a Pareto Analysis, the manager is able to focus on training as an issue, rather than spreading effort over training, taking on new staff members, and possibly installing a new computer system.

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2. Flowcharts 

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Flow charts are easy-to-understand diagrams showing how steps in a process fit together. This makes them useful tools for communicating how processes work, and for clearly documenting how a particular job is done. Furthermore, the act of mapping a process out in flow chart format helps you clarify your understanding of the process, and helps you think about where the process can be improved. 16

A flow chart is used to  

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Define and analyze processes; Build a step-by-step picture of the process for analysis, discussion, or communication; and Define, standardize or find areas for improvement in a process

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How to make a flow chart 

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Most flow charts are made up of three main types of symbol: Elongated circles, which signify the start or end of a process;

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Rectangles, which show instructions or actions; and

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Diamonds, which show decisions that must be made

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Within each symbol, write down what the symbol represents. This could be the start or finish of the process, the action to be taken, or the decision to be made. Symbols are connected one to the other by arrows, showing the flow of the process. 20

Points to note 

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Flow charts are simple diagrams that map out a process so that it can easily be communicated to other people. To draw a flowchart, brainstorm the tasks and decisions made during a process, and write them down in order. Then map these out in flow chart format using appropriate symbols for the start and end of a process, for actions to be taken and for decisions to be made. 21

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Finally, challenge your flow chart to make sure that it’s an accurate representation of the process, and that that it represents the most efficient way of doing the job

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3. Check sheets 

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The intent and purpose of collecting data is to either control the production process, to see the relationship between cause-andeffect, or for the continuous improvement of those processes that produce any type of defect or nonconforming product. A Check Sheet is used for the purpose of collecting data to compile in such a way as to be easily used, understood and analyzed automatically. 23

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The Check Sheet, as it is being completed, actually becomes a graphical representation of the data you are collecting, thus you do NOT need any computer software, or spreadsheet to record the data. It can be simply done with pencil and paper 24

The main functions - Checks 

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Production process distribution checks - where the distribution lies. Defective item checks - to determine what kind of defects exist in the process. Defect location checks - to determine where the common defects on a part are located. 25

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Defective cause checks - type of defect and thus validate the cause thereof. Check-up confirmation checks - final phase of assembly to check the finished product or work.

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4. Histograms 

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The common person believes that if a part is made in mass production from a machine, all of the parts will be exactly alike. The truth is that even with the best of machines and processes, no two parts are exactly the same. The product will have a main or "mean" specification limit, with plus/minus tolerance that states that as long as the part is produced within this range, to that range, it is an acceptable part.

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The object is to hit the target specification, however, that is not always totally possible. The purpose of a Histogram is to take the data that is collected from a process and then display it graphically to view how the distribution of the data, centers itself around the mean, or main specification.

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The histogram will graphically show   

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The center of the data. The spread of the data. Any data skewness (slant, bias or run at an angle). The presence of outliers (product outside the specification range). The presence of multiple modes (or peaks) within the data. 29

5. Scatter diagrams 

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The Scatter Diagram is another Quality Tool that can be used to show the relationship between "paired data", and can provide more useful information about a production process. What is meant by "paired data"? The term "cause-and-effect" relationship between two kinds of data may also refer to a relationship between one cause and another, or between one cause and several others. 30

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For example, you could consider the relationship between an ingredient and the product hardness; between the cutting speed of a blade and the variations observed in length of parts; or the relationship between the illumination levels on the production floor and the mistakes made in quality inspection of product produced.

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6. Control charts 

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Control charts are generally used in a production or manufacturing environment and are used to control, monitor and IMPROVE a process. Common causes are always present and generally attributed to machines, material and time vs. temperature.

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This normally takes a minor adjustment to the process to make the correction and return the process to a normal output. However, when making a change to the process, it should always be a MINOR change.

If a plot is observed that shows a slight deviation trend upward or downward, the "tweaking" adjustment should be a slight change, and then another observation should be made. 33

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Too often people will over-correct by making too big of an adjustment which then causes the process to dramatically shift in the other direction. For that reason, all changes to the process should be SLIGHT and GRADUAL! 34

Control Lines 

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A control chart is a graph or chart with limit lines, called control lines. There are basically three kinds of control lines: the upper control limit (UCL), the central line (actual nominal size of product), the lower control limit (LCL). 35

The Xbar & R Control Chart 

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An Xbar & R Control Chart is one that shows both the mean value ( X ), and the range ( R ). The Xbar portion of the chart mainly shows any changes in the mean value of the process, while the R portion shows any changes in the dispersion of the process. 36

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This chart is particularly useful in that it shows changes in mean value and dispersion of the process at the same time, making it a very effective method for checking abnormalities within the process; and if charted while in progress, also points out a problem in the production flow in real time mode. 37

7. Fishbone diagram  

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Cause & Effect Diagrams Identifying the Likely Causes of Problems Also called Fish or Fishbone Diagram and Ishikawa Diagrams

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Cause and Effect analysis helps you to think through causes of a problem thoroughly. Their major benefit is that they push you to consider all possible causes of the problem, rather than just the ones that are most obvious

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Conclusion.

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