An Introduction to Process Control Charts

Arul Nair

Last updated 23/07/2021

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Donald J. Wheeler, a world-renown expert in continuous improvement, having worked with W. Edwards Deming and later writing the classic book Understanding Variation once said: "Statistical Process Control is, at its heart, about getting the most from your processes. It is about the continual improvement of processes and outcomes. And it is, first and foremost, a way of thinking... with some tools attached." 

Well, this article of ours is inspired by his work in a huge way and also, we’d like to thank him for the perfect quote that helps us to put an introduction to process control charts. Why is that so? You’ll understand in a few minutes.

What is a Process Control Chart?

Process Control Charts (or what Wheeler calls "Process behavior Charts") are diagrams or graphs that plot process data or the management data (yields) in a period requested succession. It's a specific run outline. They regularly incorporate a middle line, a 3-sigma upper control limit, and a 3-sigma lower control limit. There may be 1-or 2-sigma limits attracted, also. The middle line speaks to the process mean or normal (and now and then the middle). 

As far as possible speak to the process variation and gives us what's regular or "basic reason" variety. In view of the common benchmark time frame to-period variety, those cutoff points are determined as to assist us with recognizing "sign" and "clamor." Again, these are determined... they are essential for "the voice of the process" and you don't get the opportunity to pick what the cutoff points are. On the off chance that you don't care for as far as possible or might suspect they are excessively wide, you need to improve the cycle to decrease variety and commotion, which is not quite the same as asking "what turned out badly?" in some random time span. 

As pioneers, we need to ensure we aren't burning through our time (or our workers' time) by requesting clarifications about the commotion. In case we will "what happened yesterday?" we need to ensure we are responding to a factually noteworthy sign in the information. One of those signs is a data point outside of those 3-sigma control limits.

Variation

At the point when a process is steady and in charge, as in the above model, you don't see anything yet normal cause variation. Regular cause variation results from the ordinary activity of a process or framework and it is relied upon because of the plan of the process, routine exercises, materials, and different variables. 

At the point when a solitary information point falls outside of as far as possible, something startling has happened to the process. Something out of the abnormal has made the process out of control. This is one model of exceptional cause variation. It demonstrates that it's improbable that the information point is because of clamor, haphazardness, or possibility. 

Note that process control graphs can uncover issues in any event, when the entirety of the information focuses fall inside as far as possible. In the event that the plot looks non-irregular, with the focuses showing a type of precise conduct, there may at present be something incorrectly. 

For instance, in the event that we have eight sequential information focuses above or underneath the normal, that is factually probably not going to be because of possibility. Factual techniques to identify successions or nonrandom examples can be applied to the translation of control graphs. In control measures show arbitrary deviation inside as far as possible.

The 4 Process States

Any process mainly falls into one of four states at any given time:

• The ideal state occurs when a process is in statistical control and produces 100 percent conformance to specifications or goals. The process is predictable and produces expected results.

• In the threshold state, the process is in statistical control but occasionally exhibits non-conformance at times.

• The brink of chaos state refers to a process that is not in statistical control but is not producing defects. This is usually a precursor to the last state;
• The process is out of control and is producing unpredictable non-conformance.

Each process is in one of these states at a particular point in time but doesn’t stay in that state. All processes will move toward the chaos of their own accord, over time, without due attention. Most companies only identify the need for intervention and improvement when the process moves to the out of control state. Control charts help organizations recognize process deterioration so that improvements can be applied to processes in the threshold or brink of chaos state.

Benefits of Process Control Charts

Organizations that practice continuous quality improvement use control charts to:

• Provide a simple, common language for talking about process performance and behavior

• Make informed decisions about which processes to leave alone and which to subject to an improvement cycle

• Limit the need for inspection

• Determine process capability based on past performance and trends
• Predict future performance if the system is stable and in control

• Assess the impact of process changes

• Visualize the performance of the process over time

• Create a baseline for future improvements

• Communicate the performance of a process

Implementation

There are a few basic steps to implementing a control chart.

Step 1: Define what needs to be controlled or monitored

Step 2: Determine the measurement system that will supply the data

Step 3: Establish the control limits based on some baseline data

Step 4: Collect and chart the data

Step 5: Make decisions based on the correct interpretations control chart information

Process Control Charts are mainstream with assembling associations utilizing the Lean or Six Sigma business system, however, they can be of extraordinary worth when applied to any cycle that has quantifiable results that can be followed after some time. Organizations of different types can profit by this basic, yet ground-breaking approach to picture process performances.

To be continued for Why Process Control Charts are a Roadmap to Improvement

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