7 Quality control tools

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 as per The American Society for 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.

The seven tools are:

• Cause-and-effect diagram (also known as the "fishbone" or Ishikawa diagram)
• Check sheet
• Control chart
• Histogram
• Pareto chart
• Scatter diagram
• Stratification (alternately, flow chart or run chart)

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 the vast majority of their workers and scaled back training to focus primarily on simpler methods which suffice for most quality-related issues.

The Seven Basic Tools stand in contrast to more advanced statistical methods such as survey sampling, acceptance sampling, statistical hypothesis testing, design of experiments, multivariate analysis, and various methods developed in the field of operations research.

The Project Management Institute references the Seven Basic Tools in A Guide to the Project Management Body of Knowledge as an example of a set of general tools useful for planning or controlling project quality.

1. Cause-and-effect diagram (also known as the "fishbone" or Ishikawa diagram):-

A fishbone diagram, also called a cause and effect diagram or Ishikawa diagram, is a visualization tool for categorizing the potential causes of a problem in order to identify its root causes.

Dr. Kaoru Ishikawa, a Japanese quality control expert, is credited with inventing the fishbone diagram to help employees avoid solutions that merely address the symptoms of a much larger problem.

A fishbone diagram is useful in brainstorming sessions to focus conversation. After the group has brainstormed all the possible causes for a problem, the facilitator helps the group to rate the potential causes according to their level of importance and diagram a hierarchy. The design of the diagram looks much like a skeleton of a fish. Fishbone diagrams are typically worked right to left, with each large "bone" of the fish branching out to include smaller bones containing more detail.

Fishbone diagrams are used in the "analyze" phase of SixSigma's DMAIC (define, measure, analyze, improve, control) approach to problem solving.

How to create a fish diagram:

• Create a head, which lists the problem or issue to be studied.
• Create a backbone for the fish (straight line which leads to the head).
• Identify at least four “causes” that contribute to the problem. Connect these four causes with arrows to the spine. These will create the first bones of the fish.
• Brainstorm around each “cause” to document those things that contributed to the cause. Use the 5 Whys or another questioning process such as the 4P’s (Policies, Procedures, People and Plant) to keep the conversation focused.
• Continue breaking down each cause until the root causes have been identified.

This example illustrates how a group might begin a fish diagram to identify all the possible reasons a web site went down in order to discover the root cause. 
 

2. Check sheet:-

The check sheet is a form (document) used to collect data in real time at the location where the data is generated. The data it captures can be quantitative or qualitative. When the information is quantitative, the check sheet is sometimes called a tally sheet.

The check sheet is one of the so-called Seven Basic Tools of Quality Control.

Kaoru Ishikawa identified five uses for check sheets in quality control:^[3]:30

• To check the shape of the probability distribution of a process
• To quantify defects by type
• To quantify defects by location
• To quantify defects by cause (machine, worker)
• To keep track of the completion of steps in a multistep procedure (in other words, as a checklist)

3. Control charts:- 

         Control charts are used to track whether the process is under control or out of control. In control charts,the data is collected and plotted over the time with the upper control limits and lower control limits and the mean level is identified.