What are SPC Charts & How to Use Them?

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May 06, 2022

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Processes are specifically curated to produce output, whether it’s a service or a product. Due to this property, they are bound to generate great volumes of data too. That is why the market has come up with different methods and tools to gain control over them.

One such method is SPC Charts, popularly referred to as Statistical Process Control Charts. The article will shed some light on SPC charts, their use, and their effective implementation.

What are SPC Charts?

SPC charts are popular tools businesses use to monitor and assess changes in processes. Besides this, they also determine the process’s stability. Despite their prevalent use, many ventures still fail to recognize their importance and proper usage. The article will shed some light on SPC charts, their use, and their effective implementation.

It plots all the data generated from different processes based on their time order. Such charts appear as a histogram showing different production variations. In other terms, it acts as a line chart to display recorded data at different times. Here is one of the most popular Statistical Process Control examples to better understand the concept. Imagine a furniture shop that deals in high-quality goods, which are only prepared where there is an order.

After an order is placed, the carpenters use their pre-planned raw material, gum, nails, and other goods and leave them to dry. Every item added in this process has a standard preparation time. The SPC charts will represent this average on a linear line chart, which provides insight to the carpenters about the entire procedure. The charts show three primary sub-categories, namely:

  • A CL or Central Line
  • An LCL or Lower Control Line
  • A UCL or Upper Control Line

Once the data is acquired and set, the organization starts its work according to them. In case there is any delay, they can easily identify its origin. In addition, they can assess the overall efficiency of every process after it is completed and compared. More about this assessment and its rectification in the latter part.

Before that, organizations should understand when is the ideal time to integrate Statistical Process Control charts. Generally, they are integrated to assess whether new improvements are elevating the process's performance. In addition, they are also used to assess:

  1. The variation resides in the process.
  2. The process's future performance.
  3. The process target-meeting capabilities.
  4. If the improvement strategy has sustained any gains.

How can an Organization Execute SPC Charts?

The execution of an SPC control chart requires the firm to commit to different functional boundaries. Read through to understand the procedure of effectively integrating SPC charts:

Decide on a Measurement Method

Start by deciding what data type will be collected, attribute or variable. The latter is a better choice due to its rich information quality. After the decision is undertaken, the company can start choosing a data control chart.

Decide the Time for Data Gathering and Plotting

Since an SPC chart only measures data changes over time, companies must maintain a time period to gather data. Depending on the organization's needs and resource scaling, the frequency can be daily, weekly, or even longer. It ensures whether they can address the quality standards while meeting the deadlines.

Integrate Control Units

Now, it is time to establish different types of control units. It can be done like this:

Start by calculating the standard deviation

Afterward, calculate the upper control line

UCL: average + 3 x standard deviation

After that, calculate the lower control line

LCL: average - 3 x standard deviation

Find Out of Control Data Points

After plotting every data point, companies can see different patterns using them. Finding out what these patterns are is pivotal to finding the cause of errors. There are even specific zones that can determine the patterns.

Rectify the Data Points

Once the erred data point is found, it should be marked on the chart to find out the cause. Once the investigation is concluded, organizations can opt for a suitable corrective matrix to reset target dates.

Determine Cp and Cpk

Now, it is time to calculate capability and performance to ensure the procedure is meeting the specifications. It can be done via:

  • Calculating Cp:

USL – LSL/ 6 x standard deviation

  • And calculating Cpk:

Min (CpU, CpL)

Track the Entire Process

After everything is in place, it is time to track the process. Ensure to follow a routine so that the system receives proactive responses, saving time and resources.

Appropriate Actions During & After SPC Charts Implementation

Every process has variations, and the method to address these variations depends on the findings. Nonetheless, organizations should follow some ground rules for optimal efficiency.

  1. Even if there isn't any special-cause variation and the system is operating optimally, do not restrain from seeking further improvement.
  2. If a special-cause variation arises while the system is still operating appropriately, start by identifying the cause instead of closing the system.
  3. A complete system change becomes a viable option if there are a system's operating efficiency malfunctions despite being under control. An example would be constant failures to meet targets or poor performance despite apt system control. 
  4. Under some unique scenarios, the system can falter on both the control and the operation front. In such cases, prioritize addressing the special-cause variation before making any changes in the process.

Closing Thoughts

Statistical Process Control charts are among the most crucial Six Sigma project initiation points. Due to its importance, many candidates try to complete a Six Sigma Green Belt Certification online.

It can help candidates understand processes better to find and rectify errors quickly. In addition, it improves their prediction and information gathering skills. The article has also provided viable information about the topic, which can help any organization get started with the SPC Control chart.

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