What is the meaning of quality?

The term quality indicates the meaning as "fitness for use," and the term fitness is defined by the customer itself who is using the product/services. The quality of a product/service refers to the degree to which the product satisfies the customer's expectations.

What is quality control?

Quality control is a broader term that includes several steps, including inspection and regulating the quality of future production. It indicates the procedures or ways to control output product quality obtained from a specific process within the range of standard/required output quality. Every plants/organization have a unit called as quality control department, which is responsible for controlling the quality of output product with respect to our requirements by using a feedback channel.

Importance of quality control

Suppose a production/service unit generates product/services on a massive scale, then there would be a possibility of some wastage of material, variation in the output quality from customer requirement, etc. The quality control techniques help to reduce the wastage of material and time, increase the product quality up to the customer's requirement level, increase customer satisfaction, reduce production cost and effective utilization of resources, etc.

Main components of quality control system

Generally, there are four main components of a quality management system to control the quality of a product/service to a required level, which are given below in the following steps.

Check

It is the first main component of the quality management system. It first checks or identifies the area where there is an opportunity for improvement. The product output should be monitored and measured to ensure whether they meet the expected quality or not.

Act

It is the second main component of the quality management system. It reviews the findings of the quality management system and re-evaluates both, the process through which production goes and the output product. The quality management process is started again to increase the quality of the output.

Plan

This step first, identifies the goals and baseline and assembles internal resources. Furthermore, it determines the quality standards and the requirements to meet the standards and determines what procedure will be used to ensure that the criteria are met.

Do

It is the final component of a quality management system in which it organizes the supporting documents like policies, procedures, training materials, work instruction, etc., in a proper sequence and trains the employees on new processes, and deploys a quality management system.

Types of quality control methods

There are many tools to control the quality of a product. The selection of quality control tools depends on the output product and should be obtained before inspection starts. Generally, there are seven primary tools used to control the quality of a product/service.

Checklists

It is the most basic quality control tool that requires checking off a list of items that are imperative to manufacture and sell your product. A checklist consists of items that you need to verify, check or inspect. The checklist ensures that you do not forget any important steps in product manufacturing.

Fish-bone diagram

It is a tool that visually represents what causes a specific problem, be it materials, machines, methods, or human resources. All types of causes can be analyzed simultaneously with the help of this diagram.

Control chart

The control chart is a graph that consists of a center line, an upper line (upper control limit), and a lower line (lower control limit). The changes in the process over time can be evaluated with the help of this chart and two limit lines.

Pareto chart

It is a bar graph in which the bar's length represents frequency or cost. The longest bar is arranged towards the left side and the smallest bar is arranged towards the right side of the chart.

Stratification

Instead of looking at all types of causes/factors together, stratification separates data into different categories so that you can identify the patterns of the input/output and the specific problem areas.

Histogram 

It represents a common graph that uses rectangular bars to visualize frequency distributions that indicate how often the defects occur. It looks like a bar graph in appearance. The image of a histogram is given below.

Scatter diagram

It is a diagram that is used to analyze the relation between two different variables. One variable is plotted on the horizontal axis in the diagram, whereas another variable is plotted on the vertical axis. The intersection points of these two variables represent the relation pattern.

Quality management system, quality assurance, and quality control

The term quality management system is a broader term that consists of quality assurance and quality control techniques. The term quality assurance indicates a way to prevent and correct the defects in an output product that is produced through a specific process before going into the customer's hand. Quality control represents the ways to correct defects or errors according to customer feedback/opinion.

Quality cost

Generally, there are three types of costs associated with the quality of a product. The basic details about these costs are given below in the following steps.

Failure cost

It is the cost of producing defective parts within the organization. The failure cost is of two types which are internal failure cost and external failure cost.

Appraisal cost

It is the cost associated with the finding and searching of the defective parts within the production system. Sometimes this cost is valued more and sometimes valued less depending on the types of defects.

Preventive cost

The expenditure made to minimize the failure and appraisal cost is termed as preventive cost. In other words, to prevent or minimize the defects, the amount of money required is referred to as preventive cost.

Value of quality

The returns obtained by an organization directly or indirectly due to good quality of their product are referred to as the value of quality. Every organization wants to maximize the value of quality by producing a good quality product.

The control chart is a graph that consists of a center line, an upper line (upper control limit), and a lower line (lower control limit). The changes of process over time can be evaluated with the help of this chart. It is divided into two types one is the variable type, and another is the attribute type. The variable type is further divided into two mean types and range types. The P-chart and C-chart are two types of attribute type charts.

Types of control charts

Control charts are divided into two types. One is the variable type, and another is the attribute type. The variable type is further divided into two types which are mean types and range types. The P-chart and C-chart are two types of attribute type charts.

Variable type

These charts are applied to the data that follows continuous distribution and can be measured on a continuous scale, for example, time, distance, weight, temp, etc. These data can be measured in fractions or decimals and are assumed to follow a normal distribution.

Attribute type

These types of data are counted and can not have fractions or decimals. These data arise while determining the presence or absence of something like success or failure, good or bad, presence or absence, defective or non-defective, etc. These data are discontinuous and assumed to follow a binomial distribution.

Mean chart

It is a type of chart that shows the centering of a process or in other words, it shows the variation in the average of a sample.

The expression of the sample mean can be represented as,

$\overline{x}=\frac{x_1+x_2+x_3+....+x_n}{N}$

Here, $\overline{x}$ represents the sample mean, $N$ represents the total number of samples and$x_1$,$x_2$ and $x_3$ represent different number of samples. 

Range chart

It is a type of chart that monitors the dispersion or variation of the process. The range chart shows the measure of the spreads of a sample.

Common Mistakes

• Students sometimes get confused regarding the difference between quality control and quality assurance. However, quality assurance refers to preventing defects in the output product before going into the customer's hands, and quality control refers to the ways to control the quality of the output product with the help of customer opinion/feedback.
• Sometimes, students also get confused about the difference between the variable and attribute types of control charts. However, variable type of chart follows continuous distributions whereas attribute type of chart follows the discontinuous distribution.
• The student also gets confused about the difference between value engineering and value analysis. However, value engineering refers to the way to reduce the cost of a product without any compromise with product quality before production, whereas the term value analysis refers to the way to reduce the cost of a product that is already in production without any compromise with product quality.

Context and Applications

There is a wide range of applications of quality control methods that are given below in the following steps.

• In forecasting
• In inventory management
• In tax filling process
• In skip tracing
• In managing costs of consultants
• In production organization (automobile, aircraft etc.)

Quality control methods is very significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Civil Engineering
• Master of Technology in Mechanical and Civil Engineering
• Doctor of Philosophy in Mechanical Engineering
• Diploma in Mechanical and Civil Engineering

Related concepts

• Quality inspection
• Value engineering
• Value analysis
• Quality assurance
• Optimum quality level
• Causes of variation
• Six sigma standard

Practice Problems

Q 1. The terms QA and QC stand for:

a. Quality acceptance and quality control

b. Quality assurance and quality control

c. Quality adjustment and quality complement

d. Quality assurance and queuing control

Correct option: (b)

Q 2. In the context of the terms QA and QC, which of the following is correct?

a. QC is an integral part of QA

b. QA is an integral part of QC

c. QA and QC are independent of each other

d. None of these

Correct option: (a)

Q 3. An example of quality assurance is

a. Verification

b. Validation

c. Documentation

d. All of these

Correct option: (a)

Q 4. Which of the following is correct regarding process control carried out?

a. Before production control

b. After production control

c. during production

d. None of these

Correct option: (c)

Q 5. Which of the following option is correct regarding variable types of the chart?

a. Mean chart

b. P-chart

c. C-chart

d. All of these

Correct option: (a)