Why SPC Study Needs to be Conducted?

A goal is not always meant to be reached; it often serves simply as something to aim at.” –Bruce Lee


There is always a reason why we do anything. When we choose to eat sweets even though we are planning to reduce weight, there is a certain Why for it. When we wake up early in the morning to do yoga and meditation, there is a Why for it. Everything we do in life has a reason and purpose (may be meaningful or not!)

Content: SPC: Why SPC Study?

  1. What is the purpose of a variable control chart?
  2. Where SPC study needs to be conducted?
  3. Where SPC not be done?
  1. Conclusion


Walter Shewhart and W Edward Deming have done tremendous work to understand the variation in the process and its causes through statistical studies (mean, range, UCL, LCL, Cp, Cpk, Pp, Ppk, p, np, c, u). The understanding of these variations is important to improve quality, reduce cost and increase productivity (QCD).

There are many logical and justified reasons to conduct SPC studies and there are a few reasons why we should not conduct the statistical study. When an SPC study is conducted, it helps the organization to reduce costs, improve productivity and enhance quality.

Once you go through the article, you will understand the purpose of the SPC study, why it needs to be done, where it is not needed and how to make its best use.

Read More: http://bit.ly/VariableAttributeControlChart

Definition: SPC: AIAG Manual 2nd Edition

SPC (Statistical Process Control): It is a collection of statistical techniques intended to understand the behaviour of a process.

Process Capability: It is determined by the variation that comes from the common causes. It generally represents the best performance of the process itself. This is demonstrated when the process is being operated in a state of statistical control regardless of the specification.

Common Cause: Variations that are consistently acting on the process. Produce a stable and repeatable distribution over time (in a state of statistical control)

Special Cause (Assignable causes): Variations that affect only some part of the process output. Often intermittent and unpredictable.

Variable Chart: It explains the process data in terms of its process variation, piece to piece variation, and its process average.

Specification Limits are the targets set for the process/product by customer or market performance or internal target. In short, it is the intended result on the metric that is measured. They have discrete values and can be counted for recording and analysis.

Control Limits are the indicators of the variation in the performance of the process. It is the actual values that the process is operating on. It is the real-time value.

Read More: http://bit.ly/CommonSpecialCause

Detailed Information

Dr Walter Shewhart developed the first control chart in the 1920s to describe common and special causes for Detection (tolerates waste) and Prevention (avoid waste)

The key objective of implementing statistical techniques is to

  • Improve quality.
  • Increase productivity.
  • Reduce cost.

There is a fundamental law of nature that no two products or characteristics are exactly alike as processes contain many sources of variation. The differences may be immeasurably small or large, but they will always be there.

There are 2 key types of causes for the variation in the process.

  1. Common Cause: Variations that are consistently acting on the process. Produce a stable and repeatable distribution over time (in a state of statistical control)
  2. Special Cause (Assignable causes): Variations that affect only some part of the process output. Often intermittent and unpredictable.

Read More: http://bit.ly/AttributeCharts

The statistical study is built on the strategy of prevention. Based on the statistical study, there can be 3 key types of variation in the process

  1. Location (mean/central value)
  2. Spread (width/span)
  3. Shape (Pattern of variation-skewed, asymmetrical etc.)

The key goal is to maintain the location to the target value with minimal variability.

Read More: http://bit.ly/SPCandMSA

SPC charts were initially developed by Dr Walter A Shewhart in the 1920s. However, they were made popular by Dr W Edwards Deming when he introduced the concept to the Japanese industry after World War II. Nowadays, SPC charts have been incorporated by organizations around the world as one of the primary tools to monitor and improve the control of a process.

An SPC chart is used to study the changes in the process over time. All the data generated from the process are plotted in time order.

Read More: https://bit.ly/SPCControlChart

Why SPC Study is needed?

The SPC process is implemented to move a company from detection-based to prevention-based quality controls. By monitoring the performance of a process in real time the operator can detect trends or changes in the process before they result in non-conforming products and scrap.

Read More: http://bit.ly/DifferenceMeanMedianMode

Where does SPC need to be Done?

  • Customer-specific requirement (Supplier manual)
  • Where special characteristics are specified (organization or customer) as one of the control
  • A new manufacturing process was added and it has special characteristics (Clause
  • Any modification in the existing manufacturing process
  • Job set up (Clause
  • When a new machine is purchased and installed
  • After a major breakdown or overhauling of the machine
  • Any major customer complaint or rejection
  • To reduce the internal rejection (rework, repair, scrap, segregation)
  • Analysis and Evaluation (9.1.3: Notes)
  • Incoming inspection data from the supplier (Clause 8.6.4)

 Read More: https://bit.ly/MSASWIPE

Where SPC not be done?

  • Where there is no or minimum variation in the process (special cause)
  • Where 100% inspection is done
  • Where SPC study is done automatically by the measurement system

Read More: http://bit.ly/SpecificationControlLimits


The purpose of the SPC study is to understand the process variation. Based on the mean and range chart, the user can analyse and decide the possible cause of the variation, whether it can impact the process or not, whether any action needs to be taken or not and what actions need to be taken on 6M so that process variation can remain under control in the manufacturing process.

Read More: http://bit.ly/CommonSpecialCause

Industry Challenges:

  • How often the objective of the statistical study is understanding the variation in the process rather than achieving Cp and CpK values?
  • How often control charts are selected (variable or attribute) based on the requirement and not based on the convenience of the available software?
  • Why do organizations conduct variable statistical studies even though 100% inspection is carried out?


IATF 16949 Standard

SPC AIAG Manual 2nd Edition

Industry Experts

This is the 213th article of this Quality Management series. Every weekend, you will find useful information that will make your Management System journey Productive. Please share it with your colleagues too.

In the words of Albert Einstein, “The important thing is never to stop questioning.” I invite you to ask anything about the above subject. Questions and answers are the lifeblood of learning, and we are all learning. I will answer all questions to the best of my ability and promise to keep personal information confidential.

Your genuine feedback and response are extremely valuable. Please suggest topics for the coming weeks.

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