Seven QC Tools
Seven QC Tools are seven tools for analysis to achieve numerical and quantitative problem solving in quality control.
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Overview of Seven QC Tools
What are Seven QC Tools?| What is QC / QC Activity? |
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| QC stands for Quality Control. It is an activity to improve the quality of products done by employees in the field through considering management and improvement of manufacturing processes by themselves in a manufacturing industry. The typical example is "KAIZEN" in TOYOTA. |
They are the following seven tools.
- Pareto Chart
- Cause and Effect Diagram
- Check Sheet
- Graph
- Histogram
- Scatter Diagram
- Control Chart
In QC activities, members of the group work on concrete problem solving in business, consider the measures to appropriately maintain the quality, to streamline tasks, to train human resources, to reduce costs, and so on, and practice them. In addition, procedures to solve problems called QC stories are defined so that workers in the production field, who are not professional of problem solving, can analyze the current status scientifically. The tools for problem solving used in these procedures are called Seven QC Tools.
Stratification
Seven QC Tools are useful when we collect data of daily activities and analyze them to detect and solve problems, and the important concept upon data analysis is stratification. Stratification means classification of data into a couple of layers, and each layer is a subset of the population. For example, classification by time of day, by region, by worker, by product, etc. is considered as stratification.
Through stratification, we could retrieve different statuses from the same data. What layers are considered for data analysis is crucial when identifying problems.
Although stratification is just a concept, it is sometimes counted as one of Seven QC Tools. In that case, Graph and Control Chart are regarded as one tool.
Usage of Seven QC Tools in QC Activities
The table below shows what kind of tasks Seven QC Tools can be applied to in QC activities. VE means "very effective," and E means "effective."
| Seven QC Tools | |||||||
|---|---|---|---|---|---|---|---|
| Cause and Effect Diagram | Check Sheet | Pareto Chart | Graph | Histogram | Scatter Diagram | Control Chart | |
| Select the theme | E | E | VE | E | E | - | E |
| Grasp the current status | E | E | E | VE | E | - | E |
| Define goals | - | E | E | VE | - | - | E |
| Measure relations among causes and effects | VE | - | - | - | - | E | - |
| Measure past statuses and the current status | - | VE | E | VE | VE | - | VE |
| Measure by means of stratification | E | E | E | E | VE | VE | VE |
| Measure temporal variation | - | - | - | E | - | - | VE |
| Measure correlation | - | - | E | E | - | VE | - |
| Consider, evaluate, and practice countermeasures | VE | - | - | E | - | - | - |
| Check effectiveness | - | E | E | E | VE | - | VE |
| Carry out thorough standardization and control | - | VE | - | E | E | - | VE |
Pareto Chart
Overview
In Pareto Chart, the vertical axis represents proportion, and the horizontal axis represents items. Bar graphs representing the items are sorted from the left in descending order in terms of the values. In addition, the cumulative frequency distribution curve, a line connecting the cumulative percentages up to each item) is also drawn.
Purpose
By drawing Pareto Chart, we can grasp to which degree each item impacts the result. Concerning items with large impact, we can effectively manage them by applying different ways of management depending on the significance, for example by intensively managing them as important management items. In general, items of which cumulative proportions reach approximately 80% are considered as important management items. (See also ABC.)
Cause and Effect Diagram
Overview
Cause and Effect Diagram is a diagram that systematically shows, by using arrows, the relations among effects and causes that affect them. Effects are subjects to look for causes and comprises of problems and results. After improvement items are determined, we need to enumerate the factors that are considered to be related to the items and thereby clarify the relationship.
Purpose
By clarifying the relationship of factors that caused results or problems, we can clarify the subject to be improved. Dimensions must be aligned, and the ends need to be subdivided until concrete countermeasures can be taken.
Graph
Overview
Graph visually represents data with figures, such as circles and broken lines.
Purpose
We can represent data according to specific objectives, such as to grasp the proportion and trend, etc.
Check Sheet
Overview
Check Sheet is a table that lists check items to facilitate inspections.
Purpose
This enables us to execute inspections exhaustively or to grasp the total number of the entire inspections when inspecting products and equipments. In addition, results of inspections using Check Sheet can be used as data.
Histogram
Overview
Histogram represents the distribution of data by using bar graphs. To create it, we need to split the domain of data into appropriate bins, count the number of data points that fall into each bin, and aggregate the results as a table. The vertical axis represents the frequency, and the horizontal axis represents bins of data. It is also called a frequency distribution chart.
Purpose
We can grasp the characteristics of tasks or products based on the center of the data distribution and the degree of variance.
Scatter Diagram
Overview
Scatter Diagram is a diagram in which data of two items are plotted.
Purpose
This helps us grasp whether there is any relation among items and how the items are related to each other. For example, in case the plots are largely distributed, we can say that there is no strong correlation between the two items.
Control Chart
Overview
Control Chart shows the trend of an item and indicates whether the item is under control or not. We can conclude that the item is under control as long as the values stay between the upper control limit (UCL) and lower control limit (LCL), allowing the variance to a certain extent.
Purpose
By grasping the trend, we can grasp the indication of an anomalous situation proactively and respond to it.
New Seven QC Tools
As the idea of quality control has been expanding into sales, research and development, services and so on in addition to the production field, it has become more and more necessary to analyze language data, such as complaints from customers, etc. Then, New Seven QC Tools are invented.
Specifically, Affinity Diagram, Relations Diagram, Tree Diagram, Matrix Diagram, Matrix Data Analysis, Arrow Diagram, and PDPC (Process Decision Program Charts) are included.
