Monitoring Process Output using Statistical Process Control Method

One of the most effective ways to monitor a key process output performance is using statistical process control (SPC) chart.  This method was invented by Dr Shewart nearly a century ago and it is the most frequent used process control method with some enhancement over times.  Control chart is actually a run chart with a calculated upper, lower control limit and process average from the actual key process output. The control limit must be calculated under the influence of stable common cause variations.  The 3 lines,  upper and lower control limits and average will be used as a guide post to show the presence of  special cause variation in the process which required immediate attention.  The interpretation of  the control chart is available in my previous article (  In order to have an effective SPC to monitor the process,  manufacturers  must  create a proper procedure on how to manage SPC implementation.

After auditing more than 100 suppliers’ sites which produce electronics parts (first and subtier) across the globe, I had NOT seen a decent SPC procedure yet.  Some companies does not even had SPC procedure and some companies SPC procedure only contain  text book  information on the type of control chart and how to plot control chart with calculation of control limits.   SPC procedure is NOT about how to plot control chart, it is should be about how to plan, implement and manage SPC within the process.  Each site/company should establish its own SPC procedure and not make copycat procedure from SPC textbook.

The table below shows some of the recommended content for SPC procedure in detail :-

Identify person in charge of SPC.
There must a dedicated department or at least dedicated team who is responsible for the implementation of SPC
SPC training for employees
Outline the SPC training curriculum for different level of employee in the company such as operator, technician, engineer and even management.
Select the parameter needs to be controlled

The most effective method to determine which process output parameter should be monitored and controlled would be through failure mode effect analysis (FMEA) technique.  FMEA is a systematic prediction use to identify potential failure which will impact customer and what are controls needs to minimize or eliminate those failure risk.  There also other method which include product mapping, brain storming etc. 
Setting up of control chart, chart selection, calculation of control limit, rationale subgrouping.
Select the most suitable type of chart (attribute or variable chart), rationale subgrouping by category (either by machine or line or tooling) and subgroup size follow by control limit calculation.  
There were a few good article rationale subgrouping by Dr DJ Wheeler in the internet
The  SPC control point must updated into the process management plan
Manage control limit

The responsible SPC person must be able to determine when to fix a control limit according to the process nature.   Normally it is recommended to study the trend of 100 subgroup points under the influence of common cause variations before fixing the control limit.
Once control limit is fixed, there should NOT be any revision on control limit done unless there is major improvement in either one or a few process input. 
Define out of control (OOC) rules

Not one manufacturing process in the world are able to use all the 7 Western Electric rules as it would be too complicated to control the process and there will be too many false alarm.  Normally I would recommend companies to use only 2-3 rules to avoid complication
Reaction plan when there is OOC
If there out of control trend per the company define out of control rules, there should be an investigation conducted to check if there is presence of special cause.  Efforts should be taken to eliminate unwanted special cause and to bring the process under the influence of only common cause.  There must be clear ownership to drive the problem to closure preferably through the company correction action request system.  Refer to my previous article on corrective action (
Review the effectiveness SPC chart
Check and balance to ensure SPC s implemented correctly and effective
There should be review conducted monthly or quarterly to ensure the effectiveness of the SPC :-
False alarm are  within preassigned goal for false alarm
SPC chart is effective to catch special cause defect, through correlation of the any special cause which happen. 

Ironically there are many companies create SPC chart just to full fill customer requirement of using SPC chart to monitor the process.  Upon a closer look at the chart, there are so many faults associated to the control chart such as:-

  1. Control limit are actually spec limit, 
  2. Out of control trends/points not investigated, 
  3. Chart is show cyclic trend of up and down due to wrong subgroup category
You can gain more insights on how to use SPC as an effective process monitoring system by taking this course : - 
Click on this image for course URL 👇

If a SPC chart does NOT serve its purpose to detect a special cause variation, then it would be better to just remove the chart totally rather than to waste resources to maintain and print the chart which does not bring any value!

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