What is SPC?

Statistical Process Control (SPC) is an approach that helps to monitor and control the output of a process by assessing the stability of the process and the type of variation that is present. SPC charts can aid decisions about whether a process is ‘in-control’ and whether or not an adjustment is necessary.

Meaning of SPC

  • Statistics: Statistics is a science that deals with the collection, summarization, analysis, and drawing of information from the data.
  • Process: It converts input resources into the desired output (goods or services) with a combination of people, materials, methods, and machines, as well as measurements.
  • Control: System, policies, and procedures in place so the overall output meets the requirement.

Why use Statistical Process Control

Today companies face increasing competition and operational costs, including raw materials increasing. So, it is beneficial for organizations to have control over their operation.

Organizations must try to continuously improve quality, efficiency, and cost reduction. Many organizations still follow inspection after production for quality-related issues.

SPC helps companies to move towards prevention-based quality control instead of detection-based quality controls. By monitoring SPC graphs, organizations can easily predict the behavior of the process.

How to Perform SPC

  1. Identify the processes:

Firstly, identify the key process that impacts the output of the product or the process that is very critical to the customer. For example, plate thickness impacts the product’s performance in a manufacturing company, then consider the plate manufacturing process.

  1. Determine measurable attributes of the process:

Secondly, identify the attributes that need to be measured during production. For example, consider the plate thickness as a measurable attribute.

  1. Determine the measurement method and perform Gage R&R:

Thirdly, create a measurement method work instructions or procedure, including the measuring instrument. For example, consider a thickness gauge to measure the thickness and create an appropriate measuring procedure. Perform Gage Repeatability and Reproducibility (Gage R & R) to define the amount of variation in the measurement data due to the measurement system.

  1. Develop a subgroup strategy and sampling plan:

Fourthly, determine the subgroup size based on the product’s criticality and determine the sampling size and frequency. For example, collect 20 sets of plate thicknesses in a time sequence with a subgroup size of 4.

  1. Collect the data and plot the SPC chart:

Then, collect the data per sample size and select an appropriate SPC chart based on data type (Continuous or Discrete) and subgroup size. For Example, for plate thicknesses with a subgroup size of 4, select X-bar -R chart.

  1. Describe the natural variation of attributes:

Next, calculate the control limits. From the above example, calculate the upper control limit (UCL) and lower control limit (LCL) for both X-bar Ranges.

  1. Monitor process variation:

Finally, interpret the control chart and check whether any point is out of control and the pattern. Example: check the X-bar R chart if the process is not in control, then identify the assignable cause(s) and address the issue. This is an ongoing process, so monitor the process variation.

When should you use SPC?

There are two key uses of SPC charts, as follows:

1) For Historical Analysis: An SPC chart can be used to analyse the past performance of a process, in order to assess if it was statistically in control (stable) or not.

2) For Ongoing Control: An SPC chart can also be used to monitor the output of a process in real time, in order to detect (as soon as possible) if the process has become ‘out of control’ (i.e. if it has statistically changed in some way).

So, in summary, SPC charts:

  • can be used to analyse if a process is statistically ‘in control’ (i.e. stable).
  • incorporate control limits and statistical tests to analyse each process result.
  • do not assess whether a process is capable or within specification.
  • can be used to analyse historical performance or monitor an ongoing process in real time.

referring documents:

  1. https://sixsigmastudyguide.com/statistical-process-control-spc/
  2. https://opexresources.com/introduction-statistical-process-control/

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