The seven techniques of quality control are commonly used statistical management methods, also known as primary statistical management methods. It mainly includes the so-called QC seven tools such as control charts, cause and effect diagrams, correlation diagrams, permutation diagrams, statistical analysis tables, data hierarchy, scatter diagrams and so on.
(a) Statistical analysis table
Statistical analysis table is the use of statistical tables to organize the data and preliminary analysis of the cause of a tool, the format can be a variety of ways, this method is more single, but practical and effective.
(ii) data stratification method
Data stratification method is the same nature of the data collected under the same conditions are summarized together for comparative analysis. Because in actual production, the quality of many factors affecting the change if not to distinguish between these difficult factors, it is difficult to derive the law of change. Data stratification can be carried out in a variety of ways according to the actual situation. For example, according to different times, different shifts for stratification, according to the use of equipment types for stratification, according to the raw material feeding time, raw material composition for stratification, according to the inspection means, the use of conditions for stratification, according to the different defects in the project for stratification, and so on. The data stratification method is often used in conjunction with the statistical analysis table mentioned above.
The application of data stratification, mainly a systematic concept, that is, in order to be quite complex information processing, you have to know how to put these data to a systematic and purposeful to be divided into categories of generalization and statistics.
Scientific management emphasizes the management techniques to make up for the lack of management by experience and visual judgment. In addition to the establishment of correct concepts, this management technique requires the use of data in order to analyze work and take correct measures.
How to establish the original data and the purpose of these data according to the need for the collection of statistics, but also a lot of quality control techniques is the most basic work.
For example: China's aviation market in recent years with the opening up of the increasingly fierce competition, airlines in order to fight for the market in addition to strengthening a variety of measures, but also in the quality of service efforts. We can also often see customer satisfaction surveys on the plane. This survey is conducted through a questionnaire. The design of the questionnaire is usually divided into the service quality on the ground and the service quality on the airplane. On the ground, it is divided into booking and waiting; on the plane, it is divided into attitude of flight attendants, food and beverage, and hygiene. Through these surveys, these data can be summarized and we can find out where to improve the quality of service.
(3) Ranking Chart (Plato)
Ranking Chart is also known as Plato, named after the 19th century Italian economist Plato, the inventor of the chart. Plato first used the permutation diagram to analyze the distribution of social wealth, he found that at that time 80% of Italy's wealth was concentrated in the hands of 20% of the people, and later people found that many occasions obeyed this law, so called Pareto's law. Later, Dr. Juran, an American quality management expert, used Plato's statistical chart to extend it for quality management. Ranking chart is a tool to analyze and find the main factors affecting the quality of the original factors, the form of double-rectangular coordinate chart, the left vertical coordinate indicates the frequency (such as the number of pieces of money, etc.), the right vertical coordinate indicates the frequency (such as a percentage of the indicated). Folding line indicates the cumulative frequency, the horizontal coordinate indicates the factors affecting the quality, according to the size of the degree of influence (i.e., the frequency of occurrence of how much) from left to right. By observing and analyzing the ranking chart, the main original factors affecting quality can be captured. This method is actually not only in the quality management, in many other management work, such as in inventory management, are very useful.
In the quality management process, to solve a lot of problems, but often do not know where to start, but in fact most of the problems, as long as you can find a few of the greater impact of the cause, and to deal with and control, you can solve the problem of more than 80%. Plato is based on the collection of data, the causes of defects, the phenomenon of defective conditions, systematically to the project category (layer) classification, calculate the data generated by each project category (such as defective rate, the amount of loss) and the proportion, and then in accordance with the size of the sequential arrangement, plus the cumulative value of the graph.
In the factory or office, the inefficiency, defects, defective products and other losses according to its cause or phenomenon, can also be converted to the loss of more than 80% of the amount of the project to be dealt with, which is known as the Platonic analysis.
Plato's use is based on the premise of the item (phenomenon) of the stratification method, and can be drawn as a plato's chart only after the statistical table has been adjusted in accordance with the order.
Steps of Plato's analysis;
(1) Classify the things to be dealt with in terms of conditions (phenomena) or causes.
(2) Although the vertical axis can represent the number of pieces, it is better to express the amount of money is stronger.
(3) Decide the period of data collection, from when to when, as the basis of Plato's data, as regularly as possible between the periods.
(4) Arrange the items on the horizontal axis from left to right according to their size.
(5) Draw a bar graph.
(6) Connect the cumulative curves.
Plato method (focus control method), provides us in the situation of not being able to cover everything, to catch the important things, the key things, and these important things are not based on intuitive judgment, but there is a data base, and graphically to enhance the representation. That is, the stratification method provides the statistical basis, and Plato's law can help us to catch the critical things.
(D) Cause and Effect Diagram
A cause and effect diagram is a diagram that uses the effect as a characteristic and the cause as a factor, with an arrow linking them together to indicate the cause and effect relationship. Cause and effect analysis chart is a fully mobilized staff brainstorming, investigate the causes, brainstorming a good way, but also particularly suitable for the implementation of democratic management of quality in the work group. When there is a quality problem, not clear reasons, can be mobilized for the problem to look for possible causes, so that everyone speaks freely, all possible causes are listed.
The so-called cause and effect analysis diagram is to cause a result of the many causes, in a systematic way graphical, that is, to express the relationship between the results (characteristics) and causes (factors). It is shaped like a fishbone, also known as a fishbone diagram.
There must be a reason for the formation of a certain result, and the graphical method should be utilized to find out the cause. The first to put forward this concept is the Japanese quality control authority Dr. Ishikawa Xin, so the characteristics of the cause of the map is also known as [Ishikawa diagram>. Cause and effect analysis charts can be used in various stages of general management and work improvement, especially in the early stages of establishing awareness, making it easy to clarify the cause of the problem, so as to design steps to solve the problem.
(1) Steps in the use of Cause and Effect Diagrams
Step 1: Gather the people involved.
Gather the people who are related to the problem and have experience in it, preferably 4-10 in number.
Step 2: Hang a large piece of white paper and prepare 2-3 color pens.
Step 3: The assembled people will speak on the causes affecting the problem, and the contents of their speeches will be recorded on the chart, without criticizing or questioning in the middle. (Brainstorming method)
Step 4: It will take about 1 hour to collect 20-30 causes.
Step 5: On the reasons collected, what has the greatest impact, and then by the big turn to speak, after consultation, that the greater impact on the circle on the red circle.
Step 6: The same as step 5, for those who have circled a red circle, if you think it is the most important can be circled two or three times.
Step 7: Re-draw a picture of the cause, not on the circle to be removed, the more the number of circles listed as the highest priority.
Cause and effect analysis of the map provides a tool to capture the important reasons, so participants should be included in this work with experience in order to ease the Qin effect.
(2) The Use of Cause and Effect Diagrams and Plato
The creation of a Plato requires the creation of a statistical table of the required purpose in a hierarchical manner. The purpose of creating a Plato is to grasp the important few items that have a large impact on the overall situation. Then, using the characteristic cause diagram, the causes of the formation of these items are discussed one by one, and countermeasures are taken to improve them. Therefore, the cause and effect analysis chart can be used alone or in conjunction with Plato.
(3) Re-analysis of Cause and Effect Diagram
The root cause of a problem can be investigated in order to solve the problem at its root. Formation of the main cause of the problem to find out, and then the experimental design method of experimental analysis, proposed specific experimental methods, to find out the best way to work, the problem may be able to completely solve the problem, which is to solve the problem, but also to prevent the problem.
Any person, any enterprise has its pursuit of the goal, but in the pursuit of the goal of the process, there will always be many tangible and intangible obstacles, and what are these obstacles, these obstacles to the formation of these obstacles, these obstacles to how to crack the problem, is the main concept of the cause of analyzing the map method.
A manager, in his management work within the scope of the pursuit of the goal, if they are specifically summarized, we can learn from the project is not a lot. However, for each project, there are primary and secondary reasons that affect the achievement of the goal, and these reasons are the variables that prevent you from achieving your work.
How to list the projects you are pursuing, and organize the major and minor causes that affect the achievement of each project, and use a cause-and-effect diagram to represent them, as well as to systematically reinforce them, will make your management work more convenient.
Likewise, with these cause-and-effect diagrams, even if a problem occurs, the process of analyzing the problem will be faster and more reliable.
(v) Histograms
Histograms, also known as bar charts, are a primary tool for representing changes in data. With the histogram can be messy information, analyze the regularity, more intuitively see the distribution of product quality characteristics of the state, for the central value of the information or the distribution of the situation at a glance, to facilitate the judgment of its overall quality distribution. In the production of histograms, involving some statistical concepts, first of all, the data should be grouped, so how to reasonably group is one of the key issues. Grouping is usually carried out according to the principle of equal group spacing two key figures are the number of groups and group spacing.
(F) Scatterplot
Scatterplot, also known as correlation plot, it is the two variables that may be related to the data with a point drawn on a coordinate graph, used to indicate whether there is a correlation between a group of paired data. This kind of paired data may be characteristic of a cause, characteristic of a characteristic, cause of a cause of the relationship. The correlation between two variables is determined by observing and analyzing them. This kind of problem is also common in actual production, such as the relationship between the quenching temperature and the hardness of the workpiece during heat treatment, the relationship between the content of a certain element in the material and the strength of the material. This relationship exists, but it is difficult to use the exact formula or functional relationship, in this case with the correlation diagram to analyze is very convenient. Assuming that there is a pair of variables x and y, x represents a certain kind of influencing factors, y represents a certain quality characteristic value, through the experiment or collected data of x and y, can be expressed on the coordinate graph with points, according to the distribution characteristics of the points, you can judge the correlation of x and y.
In our life and work, many phenomena and causes, some of which are regularly associated, some of which are irregularly associated. To understand it, we can use scatterplot statistics to determine the correlation between them.
(VII) control chart
Control charts are also known as control charts. By the United States of America's Bell Telephone Laboratory Hugh Hart (W.A. Shewhart) Dr. first proposed in 1924 control charts used, control charts have been an important tool for scientific management, especially in quality management has become an indispensable management tool. It is a kind of chart with control boundaries, which is used to distinguish whether the causes of quality fluctuations are accidental or systematic, and can provide information on the existence of systematic causes, so as to judge whether the production process is in a controlled state. Control charts can be divided into two categories according to its use, one is for the analysis of control charts, control charts to analyze the production process of the quality characteristics of the value of the changes in the process to see whether the stability of the controlled state; and another is for the management of the control charts, mainly used to find out whether the production process of the anomalies, in order to prevent the production of defective products.
Statistical management is an effective tool for quality control, but in the application of the following issues must be noted, otherwise it will not have the desired effect. These problems are: 1) data errors. Data errors may be caused by two reasons, one is the use of human error data, the second is due to the lack of true mastery of statistical methods; 2 ) data collection methods are not correct. If the sampling method itself is wrong, then the subsequent analysis of the correct method is useless; 3) data recording, transcription errors; 4) the treatment of outliers. Usually in the production process to obtain the data always contains some outliers, they will lead to analysis results are wrong.