Statistical Quality Control- The Ultimate Guide

Statistical Quality Control- The Ultimate Guide

You know Quality matters a lot in this Competitive World. You have also seen New Startups like Zomato, Swiggy. What they did, Any guesses??  They have also used the same concept of Statistical Quality Control.

They had entered into the Indian Market with their Quality Services, Product whatever they have. But they keep one thing in mind that they will not earn profit instantly.

But they have not compromised with the qualities. The customer is the King of the Market and Customer wants Quality. You know in Ancient Times, there are rules of Kings and Queens.

What they say, people have to do that work. If One will not do that work, others will do. So, Statistical Quality Control is the same as that.

In the Same Way, Customers has also so many alternatives avaliable with them.

If any Company willn’t give Quality product/service to the Customer, then Customer can buy from other vendors/Companies also.

I believe You have understood the example of Statistical Quality Control .

So, Let’s know the meaning of Statistical Quality Control??

What is Statistical Quality Control??

Statistical Quality Control

SQC refers to improve the quality of the product by using statistical methods. It involves the application of the theory of probability and sampling in order to solve the problems concerning the quality of the products.

It can be measurable( in quantities) and countable( the no. of defects in the piece of product).

Now Come to know it’s meaning said by Big Statisticians.

Definition of Statistical Quality Control

Statistical Quality Control is applied by taking the example and drawing the conclusions. That can be done by applying statistical tools.”

Statistical Quality Control PDF and PPT

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Where was Statistical Quality Control Originated??

Where was Statistical Quality Control Originated??

SQC was originated firstly in the United States of America. Walter A. Shewhart of the Bell Telephone Laboratories has introduced this Concept Statistical Quality Control in 1924. He is the one who has come with this Concept.

But Quality was the concern from a long time. But in Previous time, the Customer has to purchase those goods which are available in the market.

That was known as Production Concept. Customers are also concerned about quality at that time. But at that time, Customers are not available with alternative sellers.

So, many approaches came and go. But the time has changed now.

Now, Firm produce goods according to the needs of the Customer which well known as Customer Oriented Approach.

Now, Customers are available with the other options to buy from someone else.. Today, so many seminars are conducted for the firms to know the Needs of Customers.

This clearly shows the importance of Quality in the Product.

So, You have understood the meaning and who has introduced the Statistical Quality Control.

But How does Statistical Quality Control leads to improvements in the Business??

How does Statistical Quality Control leads to the Improvements in the Business??

Today Every Company either they are giving products or services or their size is small or large, they all are using Statistical Quality Control. But How they determine the variations in the Quality??

In SQC, Firm ascertain the variations in the quality due to Chance Causes and Assignable Causes.

I am not saying that firm read the theoretical aspects. But they apply the same concept.

So, now You will be thinking What are these causes??


Chance Causes are those causes which are very difficult to identify. You can also say that the variations are very less. To check the Variations in this cause is Uneconomical.

For Example-  A Milk factory produces 500ml milk packet. Quality Inspection Officer checked the quality. He found the number of milk 499.7ml which is acceptable. In this, Officer can’t blame for this variation to anyone.

Assignable Causes are those causes which we can easily identify the variations. The change in the Variation can be due to-

  • Worker’s Performance
  • Machine’s Performance
  • Variations in Material.

For Example-  The Same Quality Inspection Officer of Milk Factory. In this time, He found the no. of milk 490, 492 ml which is not acceptable. In this, the Officer will try to check by whom’s mistake this variation has occurred.

I can explain this by giving you an example. You can also make this example in your Exam. This will help you in getting more marks.

Chance and Assignable Causes

I believe You’ve pretty much understood by this graph. This is the Same Milk Packet example of 500ml.

In this, Quality Inspection Officer has to ascertain whether the variations in the quality of the product is due to Chance causes or Assignable Causes and How they check this??

If the Process is in the statistical control between 491-510, then it is known as Chance Causes and If the Process if between 400-490 or 511-600 , then it is known as Assignable Causes.

This is about Controlling the Quality. But If You Want to know how to manage the Quality for a Firm, then you can click here.

Now, It’s time to know the techniques of Statistical Quality Control.

Statistical Quality Control Techniques

Statistical Quality Control Techniques

As it is clearly showing in this image that the Techniques of Statistical Quality Control. It is divided into two major parts- Process Control and Product Control.

So, let’s discuss this one by one.

1. Process Control-  In Process Control, Inspection Officer control the quality of the goods while they are in the manufacturing process.

The benefit of this method is to detect the problems before actually converting into the finished goods.

While checking the products quality, they have to determine whether the process in under control or not. For that, they Prepare Statistical Quality Control Charts.

Statistical Quality Control Charts

Statisrical Quality Control Charts is a graphical tool used to determine whether the process is under control or not. When the Process is under control, it will be known as Chance causes and when it is not under control, it will be known as Assignable Causes.

Statistical Quality Control Charts

These Controls can be Measurable or Countable. For Measurable, it will be known as Control Chart for Variables. For Countable, it will be known as Control Chart for Attributes which are its further sub- classifications.


Control Chart for Variables-

Control Charts for Variables applies when there is an involvement of measurable terms. It is also categorized into two more Charts that are as follows-

a. x Chart- It is a Statistical Quality Control Chart which applies when the output of a process is measures in terms of the Mean value (x̅) of the variable.

In this, the limits are-

X Bar Chart in Statistical Quality Control


b. R Chart-This chart is useful for controlling the variations in the dispersion or the variability of the quality standards of the products in the production process.

Range= Highest Value- Lowest Value

R( Range Chart) in Statistical Quality Control


Control Chart for Attributes-

For Countable terms, Control Chart for Attributes is useful like the No. of Defects in a piece of Cloth. It is also segregated into three more charts.

a. P Chart- This Control Chart is productive for controlling the quality standards in the average fraction Defective of products in a process. It came into use when there is a classification of Defective and Non-Defective Items.

The CL, UCL and LCL are as follows-

P Chart in SQC



b. np Chart- This Chart is useful for controlling the quality standards for the attributes in the process. It applies where Sample size is equal and it requires to plot the no. of defectives in sample instead of Fraction Defective.

The Control Limits are as follows-

np Chart in SQC


c. C Chart-  This  Statistical Quality Control Chart is having relevance to control the no. of defects per unit like Piece of Cloth, Piece of Paper which may contain more than one defect. Here, the Inspection Unit in this Chart will be a single unit of product.

The Probability of Occurrence of defects tends to remain very small.

The Statistical Quality Control Charts are as following-

C Chart in Statistical Quality Control

Here, c denotes the Total No. of Defects in a Product/No. of Units.

2. Product Control- In Product Control, the Quality Inspection Officer examine the quality of the product when it is in the semi-finished form.

In this, a Statistical Procedure is used to reject or accept the lot which is known as Acceptance Sampling that is explained below-


Acceptance Sampling-
Statistical Quality Control Techniques

It is a statistical process in which the no. of Defective Items are found in a sample. It is used to determine whether the a lot should be accepted or rejected.

The Process will be like-

  • Lot Received
  • Take a Sample from the population
  • After this, a Sample is interpreted in given quality.
  • Compare with the Predetermined Quality
  • Then, Accept or Reject.

This Process can be done in 3 ways. But These ways has one similarity and That is Sampling. In these ways, Sample has been taken to accept and reject the quality of product.

Acceptance Sampling
  • Single Sampling Plan- In this method, One Sample is drawn to take the decision to Accept or reject the lot.

Here, the following denotes-

d( Defective No.)

c( Compare with Acceptance No.)

Accept if d ≤ C

Reject if d > C

  • Double Sampling Plan- In this plan, if we are not satisfied from the First Sampling, then We take two Sample.
Acceptance Sampling in Statistical Quality Comtrol

Acceptance Sampling in Statistical Quality Control


  • Multi- Stage Sampling Plan- This Plan is useful when we don’t satisfy from the two samples, then we take last sample.
Advantages of Statistical Quality Control
Advantages of Statistical Quality Control

Now If you have read the above techniques, I am sure you have been understood  how these are used. Now One question is coming in your mind.

That the techniques which We read. Is there any benefit of these techniques in the Real life??

My Answer is Yes. There are some Benefits.

So, Let’s read the advantages of Statistical Quality Control

  • Control the Quality-  Statistical Quality Control helps to control the quality of the product by comparing with the standard performance to accept or reject.
  • Eliminate Assignable Causes of Variation-  If there are variations in the product that are not easy to eliminate. Then, they eliminate those assignable causes. It can be due to-  

Worker’s Performance

Machine’s Performance

Variation in Material

  • Better Quality- By Eliminate the assignable cause, we get a very good quality product at a lower Inspection rate.
  • Useful for Both- Better Quality is helpful for both Consumers and Producers. For Consumers, they are getting Quality Product, For Producers, by giving the Quality Product , they are gaining attention from the Customers.
  • Makes Customer Quality Conscious-  When Customer will start getting quality products, they will become Quality Conscious. Then, they willn’t buy from those producers who doesn’t provide quality products.
  • Earn Goodwill- As Firm will start provide quality products to the Customers, their Goodwill will automatically increases . Every Customer remember their Brand as Quality product/Service.
  • Pressure for Further Quality Improvements-  Statistical Quality Control exerts more pressure for further quality Improvements. It can be in the way to do improvements for future in case any update comes in the product.

To know About How to manage the quality up to Top Level, then click here.

Conclusion of Statistical Quality Control
Conclusion Of Statistical Quality Control

I want to Conclude in the End that Today, Every Customer is Quality Conscious. We can face losses in starting by providing good quality products at less Prices.

But no Firm can give good Quality Product for the long time at less price. Firm will also increase the prices once capturing the market.

So, Gain the Attention of the Customers but don’t loss your Gain or Profit which is also necessary.

Thank you for paying your Attention.
If You want all the Notes of Statistical Quality Control Notes in PDF form and the slides of all the Notes in PPT form. Then, You can download from the button below-

Statistical Quality Control PDF
Statistical Quality Control PPT

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