Statistical process control (SPC)

Updated: Sep 13

What is SPC?

Statistical process control is define the use of statistical techniques to a process and production method.

"SPC is the use of Statistical techniques to analyse a process or its outputs so that appropriate action can be taken to achieve and maintain the state of control."

What is the Meaning of statistics?

The set of information derived from the sample data to estimate the process (population) are called statistics

What is the Meaning of Process?

converting an input into output by using man, machine , method,material and environment is called process.

What is the Meaning of control?

Ensuring the to make a variable to be with in the stated limit.

SPC is considered one of the 5 core tools of IATF 16949

SPC Control System

The introduction of statistical process control is stabilised the process and

reduce the variability.

SPC History

In mid of 1920 Dr. Walter. A. Shewhart developed the fundamental of SPC and the associated tools of the control chart.

Principle of SPC

The basic Principle of SPC is to reduce the variation

What is Variability?

variability is define as the net result of many factors , which are constantly affecting the process

Cause of variability:

1-Common cause

Common cause are those cause which randomly affect the system, they are always present and built in to the process it self. this is also called as "inherent cause".

2-Special Cause

Special cause are those cause which represent non-random variation in process , which can be identified to a particular cause.

special variation might be caused by one or more of the following

  • Man-Setup (Speed, Feeding), accuracy, training, Experience etc.

  • Machine- Accuracy, Calibration, wear etc.

  • Material- Different material , mixing accuracy etc.

  • Measurement- Repeatability, Precision, accuracy, calibration etc.

  • Environment- Temperature , humidity etc.

if the process is said to be "in control" , it means only common cause of variation are present.

if the process is said to be "out of control", it means that special cause of variation are present.

The process is said to be "statistical control" when all special cause of variation have eliminated and only common cause of variation remains.

Statistical process control (SPC) is a powerful collection of problem-solving

tools useful in achieving process stability and improving capability through the reduction of


SPC is one of the greatest technological developments of the twentieth century because

it is based on sound underlying principles, is easy to use, has significant impact, and can be

applied to any process. Its seven major tools are these:

1. Histogram

2. Check sheet

3. Pareto chart

4. Cause-and-effect diagram

5. Defect concentration diagram

6. Scatter diagram

Of the seven tools, the control chart is probably the most technically sophisticated

Statistical Basis of the Control Chart:

The control chart is a graphical display of a quality characteristic is measured or calculated from a sample versus the sample number or time. The chart have a center line that represents the average value of the quality characteristic corresponding to the in-control state.Two other horizontal lines, called the upper control limit (UCL) and the lower control limit (LCL)

Classification of control chart

Benefits of SPC

  • Increase the customer satisfaction by producing the more trouble free parts.

  • Decrease scrap , rework and inspection cost by controlling the process

  • improve the productivity by identifying and eliminating the cause of out-of control condition

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