7 basic quality tools
The most asked interview question for a Mechanical Engineer!
Table of Contents:
What is quality?
Quality is a subjective topic. The reason is that the requirements about a particular thing might differ from person to person or organization to organization. Due to this, there exists a lot of definitions for quality, but to put it in general terms ISO has defined quality as:
“Degree to which a set of inherent characteristics fulfils requirements”.
Seven Basic Quality tools are also called ‘The old seven’, ‘The first seven’, ‘The basic seven’.
The seven basic tools of quality is a designation given to a fixed set of graphical techniques identified as being most helpful in troubleshooting issues related to quality.
They are called basic because they are suitable for people with little formal training in statistics and because they can be used to solve the vast majority of quality-related issues.
7 basic quality tools
With the seven basic tools of quality, you can easily manage the quality of your product or process, no matter what industry you are in. The 7 basic tools are as follows:
Cause-and-effect diagram / fishbone diagram / Ishikawa diagram
As the name suggests, the diagram indeed looks like a fish with its skeleton as shown in the figure above, hence also called herringbone diagrams. It is created by Ishikawa hence also called Fishikawa.
Definition: An Ishikawa diagram is a diagram that shows the causes of an event and is often used in manufacturing and product development to outline the different steps in a process, demonstrates where quality control issues might arise, and determine which resources are required at specific times.
When to use: When identifying possible causes for a problem.
How to use / Process:
Brainstorm about and come to conclusion for effect or the problem statement.
Brainstorm the major categories of causes of the problem. :
Brainstorm "Why it happens?" and keep asking Why until you reach the root cause (5 Why) and keep adding points in the fishbone diagram.
Benefits and outcome: Solve the root cause of the problem and improve the quality.
Diagrams can become messy for complex problems.
Irrelevant potential causes can cause confusion.
Example: Imagine you own a restaurant and the customers have been complaining that the burgers are not good. The fishbone diagram for this situation will be as follows:
It is a simple tool used to collect and organize the data. The data it captures can be quantitative or qualitative. When the information is quantitative, it is also called a tally sheet.
Definition: The check sheet is a form (document) used to collect data in real-time at the location where the data is generated.
When to use:
When you want to check the shape of the probability distribution of a process.
When data can be observed and collected repeatedly by the same person or at the same location.
When collecting data on the frequency or patterns of events, problems, defects, defect location, causes (defect), type, or similar issues.
When you want to keep track of the completion of steps in a multistep procedure.
How to use / Process:
Decide on your goal or what event should be observed.
Decide on how, when, and how long the data will be collected.
Design the form and try it for some time.
Every time the problem occurs make a record on the checksheet.
Benefits and outcome: Great to do analysis on the data collected and work upon it.
Example: Below is an example for an employee who works in a garage. This checklist is a great way to keep track of the vehicles.
A control chart always has a central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. These lines are determined from historical data.
Definition: The control chart is a graph used to study how a process changes over time. Data are plotted in time order.
When to use:
When controlling ongoing processes by finding and correcting problems as they occur
When predicting the expected range of outcomes from a process
When determining whether a process is stable (in statistical control)
How to use / Process:
Choose the appropriate control chart for your data.
Determine the appropriate time period for collecting and plotting data.
Collect data, construct your chart and analyze the data.
Look for "out-of-control signals" on the control chart. When one is identified, mark it on the chart and investigate the cause. Document how you investigated, what you learned, the cause, and how it was corrected.
Benefits and outcome: By comparing current data to these lines, you can draw conclusions about whether the process variation is consistent (in control) or is unpredictable (out of control, affected by special causes of variation).
Example: Below is an example where a doctor keeps track of the number of patients in his hospital.
One of the simplest tools to do this with is a histogram, a quality tool that many of us will be familiar with from school.
Definition: A specific form of bar chart used to represent the central tendency, dispersion, and pattern of a statistical distribution.
When to use: You want to see the shape of the data’s distribution, especially when determining whether the output of a process is distributed approximately normally.
How to use it / Process:
Collect at least 50 consecutive data points from a process.
Use a histogram worksheet to set up the histogram. Draw x- and y-axes on graph paper and name them. Do not allow for spaces between bars.
For each data point, mark off one count above the appropriate bar with an X or by shading that portion of the bar.
Benefits and outcome: Analyzing whether a process can meet the customer’s requirements. Visualize the data.
Example: A teacher might create a histogram showing the points students scored in an exam, which might help her figure out how much students scored based on average points and improve her skills or get feedback.
Vilfredo Pareto, an Italian engineer, and economist, first observed the 80/20 rule in relation to population and wealth. At the beginning of the 20th century, Pareto noted that in Italy and several other European countries, 80% of the wealth was controlled by just 20% of the population.
Definition: The Pareto chart is a quality improvement tool that is based upon the Pareto principle, the principle that 80% of an outcome comes from 20% of its inputs.
When to use: When there are many problems or causes and you want to focus on the most significant.
How to use / Procedure:
Decide what categories you will use to group items.
Decide what measurement is appropriate. Decide what period of time the Pareto chart will cover.
Collect the data. Subtotal the measurements for each category. Mark it on the Chart.
Benefits and outcome: Easy to grasp the problem and narrow problem area.
Example: An outline of the chart.
The scatter diagram graphs pairs of numerical data, with one variable on each axis, to look for a relationship between them. If the variables are correlated, the points will fall along a line or curve. The better the correlation, the tighter the points will hug the line.
Definition: A scatter plot is a mechanism to understand the influence of an independent variable over a dependent variable.
When to use: When trying to determine whether the two variables are related.
How to use / Process:
Collect pairs of data where a relationship is suspected.
Draw a graph with the independent variable on the horizontal axis and the dependent variable on the vertical axis.
Look at the pattern of points to see if a relationship is obvious. Otherwise, you may use regression or correlation analysis now.
Benefits and outcome: Understand if the variable is not in control and systematically change with another variable.
Example: Below is an example of the smoothness of cut finish varying with respect to drilling speed. There is a clear increasing linear relation between them.
Stratification / flow chart / run chart
Stratification is a technique or method that can be defined in many ways.
Definition: Stratification is defined as the act of sorting data, people, and objects into distinct groups or layers. A technique used to analyze and divide a universe of data into homogeneous groups is called - Strata.
When to use: When data from a variety of sources or categories have been lumped together, the meaning of the data can be difficult to see.
How to use / Procedure: When plotting or graphing the collected data on a scatter diagram, control chart, histogram, or other analysis tools, use different marks or colors to distinguish data from various sources. Data that are distinguished in this way are said to be "stratified." Analyze the subsets of stratified data separately.
Benefits and outcome: Due to the formation of layers or classes or categories the analysis of the problem becomes easy.
Example: Below is an example of a few machines and defective pieces the machine produces.
The designation arose in postwar Japan, inspired by the seven famous weapons of Benkei. It was possibly introduced by Kaoru Ishikawa who in turn was influenced by a series of lectures W. Edwards Deming had given to Japanese engineers and scientists in 1950. At that time, companies that had set about training their workforces in statistical quality control found that the complexity of the subject intimidated most of their workers and scaled back training to focus primarily on simpler methods which suffice for most quality-related issues. Dr. Ishikawa developed the concept of quality circles. He believed that everyone should be involved in quality improvement. Quality circles provided a method of doing this. It enabled everyone to work on process improvement by suggesting ideas to improve products and processes. All quality circles were given training in the basic problem-solving tools, including the seven basic tools covered in this publication.
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