What is Control Chart and its types?

The control chart is an online graph used to study how a process changes over time. The control chart was invented by Walter A. Shewhart.

Guidelines for Control Chart:

A control chart always has a central line for the average, an upper line for the upper control limit and a lower line for the lower control limit. These lines are determined from historical data.

By comparing current data to these lines, you can conclude whether the process variation is in control or out of control.

Control Chart

When to Use a Control Chart :

  • When controlling ongoing processes by finding and correcting problems as they occur.
  • When predicting the expected range of outcomes from a process.
  • When determining whether a process is stable.
  • When analyzing patterns of process variation from special causes or common causes (within the process).
  • When determining whether your quality improvement project should aim to prevent specific problems or to make fundamental changes to the process.

Types of Control Charts:

There are two categories in the control chart. One is the variable control chart and the second is the attribute control chart.

Further in the Variable control chart, there are:

  • Average and Range chart (X – R)
  • Individual and Moving Range Chart ( X – MR / I – MR)

In the Attribute category, there are,

  • Number of defectives chart ( p / np chart)
  • Number of defects ( c / u chart)

Control Chart Basic Procedure:-

  1. Choose the appropriate control chart for your data.
  2. Determine the appropriate period for collecting and plotting data.
  3. Collect data, construct your chart and analyze the data.
  4. Look for “out-of-control signals” on the control chart. When one is identified, mark it on the chart and investigate the cause. Document how you investigated, what you learned, the cause and how it was corrected.
  5. Continue to plot data as they are generated. As each new data point is plotted, check for new out-of-control signals.

How to interpret the control chart?

Control Chart
  1. Any point out of control limit ( Out of Zone A)
  2. Seven points in a row on one side of the center line.
  3. Six points in a row continuously increasing or decreasing order.
  4. Fourteen points in a row alternating up and down.
  5. Two points out of three points in a row in the same zone A.
  6. Four points out of five points in the arrow in the same zone B.
  7. Fifteen points in a row in zone C above or below the center line.
  8. Eight points in the arrow on both sides of the center line but none in zone C.

Guidelines for eliminating Special cause variation:

  • Get timely data so that you see the effect of the assignable cause soon after it occurs.
  • As soon as you see something indicating that an assignable cause of variation has happened, search for the cause.
  • Change tools to compensate for the assignable cause.

Guidelines for reducing Common cause variation:

  • Reducing common-cause variation  usually requires making fundamental changes in your process
  • Addressing the common cause variation will improve the process performance.

You may also read about SPC, Control Charts and Histogram.

Benefits of Control Charts:

  • Predict processes out of control and out of specification limits.
  • Distinguish between specific, identifiable causes of variation.
  • Control charts allow operators to detect manufacturing problems before they occur, this greatly reduces the need for product rework or COPQ.
  • Control charts provide the early warning detection system, to make changes in the running process.
  • Can be used for statistical process control.
  • After analyzing a control chart, operators need to determine whether to “do something” (adjust in the process) or “do nothing,” (let the process run as is).

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