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An Overview Of Statistical Process Control

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					?The manufacturing process in your company is worrying you. You want to know
how to monitor and control the processes in your factory. Statistical Process Control
(SPC) offers you a tool to determine when a statistically significant change has
occurred in the process or when one is going to take place.

Statistical process control is used in many companies for a number of reasons.
Primarily, control charts and other SPC techniques are used to reduce variation and
improve manufacturing processes. It is also implemented to satisfy customer needs
and meet with certification requirements.

Remember, SPC provides a tool to identify causes of variation in the manufacturing
process. But by itself, it will not make any improvement. Hence, it is the
responsibility of the operator to use this tool and take further action. If the cause has a
positive impact on the process, it must be built into it, else eliminated.

A control chart is the most important technique of SPC. There are two types of control
charts that you could use.
? Attribute data control charts: Here, data is taken by counting the number of
defective items.
? Variable data control charts: Data is measured on a variable scale like the length,
width or height of the defect.

The latter is a more useful measure of process improvement as it helps to precisely
identify the variations.

The following is a simple process to implement statistical process control in your
factory:

? Determine the critical process parameter that needs to be monitored.
? Set up an initial control chart. This is to confirm that the process is in control.
? Collect and plot the future data on the chart.
? Now interpret the chart to find out if the process is out of control.
Bear in mind the following while using statistical process control:

? It is essential for you to first conduct a process capability study.
? Train your operators on the usage of SPC. Do not assume that the control charts
are generated by computer programs.
? Pick random samples from the process or use a sampling frequency or sample
size that allows you to identify variation in the process.
? Use the right kind of chart for the process that will result in accurate signs.
? Do not put specification limits on the control charts.
? Do not plot the data for a control chart after the process has begun.
? Review control charts and their usage on the shop floor with the operators from
time to time.
?   Do not use only control charts to satisfy customer requirements.

Books like "Understanding Statistical Process Control" and "Understanding Variation:
The Key to Managing Chaos" from are a must read for performing an effective SPC.
There are also many software packages that can make the calculations for SPC much
less burdensome. Alliance/MFG by Exact Software and COSS Manufacturing
Software from are designed to offer solutions tailored to manufacturing operations.




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