Control Charts
Introduction
A Control Chart is a tool you can use to monitor a process. It graphically depicts the average value and the upper and lower control limits (the highest and lowest values) of a process.

All processes have some form of variation. A Control Chart helps you distinguish between normal and unusual variation in a process. If you want to reduce the amount of variation in a process, you need to compare the results of the process with a standard.

Variation can exist for two reasons:

  • Common causes are flaws inherent in the design of the process.
  • Special causes are variations from standards caused by employees or by unusual circumstances or events.

Control charts for measurements

  • X-bar charts
  • R charts
  • Control charts for compliance items

  • P charts
  • c charts
  • X Bar Chart
    Monitor process location (center)

  • Decide on the quality to be measured.
  • Determine a sample size.
  • Gather 20 to 30 samples.
  • Compute the sample average for each sample.
  • Compute the sample range for each sample.
  • Determine the average sample mean for all samples.
  • Determine the average sample range (or sample standard deviation) for all samples.
  • Determine the value of A2 ( for range) or A3. (for standard deviation).
  • Compute the UCL and the LCL

    Monitor process variation

  • Decide on the quality to be measured.
  • Determine a sample size.
  • Gather 20 to 30 samples.
  • Compute the sample range for each sample.
  • Determine the average sample mean for all samples.
  • Using the size of the samples, determine the values of D3 and D4.
  • Compute the UCL and the LCL

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