Control Chart in Quality Management

The Control chart is one of the Seven Basic Quality Tools.

What is a Control Chart?

PMBOK defines Control chart as a tool that is used to determine whether or not a process is stable or has predictable performance.

Purpose of a Control Chart

The main purpose of a Control chart is it helps monitor production and other processes to see if the results are within acceptable limits, if not then are there any actions required.

Upper and Lower Control limits

Upper and Lower control limits are the allowed/acceptable maximum and minimum results of a process. They are based on the requirements. The range of values between these two limits is the acceptable range of variation of a process or results.

Upper and lower control limits are shown as “two dashed lines” on a control chart.

Project Manager and appropriate stakeholders determine the upper and lower control limits.

Mean 

Mean is the middle of the range of acceptable variation. It is indicated by a line, located in the middle of the control chart.

Specification limits

Specification limits represent the customer expectations or contractual requirements for performance and quality on the project. We get these limits as inputs from the customer. They appear either inside or outside of the control limits.

Process “Out of Control” situations

A Process is said to be “out of control” in any of these situations

  1. Assignable cause / Special cause variation – A process data point falls outside of upper or lower control limit.
  2. Rule of Seven – A group or series or of 7 or more non-random data points on one side of the mean.
    • In this situation, these 7 or more points need not be outside the control limits, but they should not be random and should be in a continuous series.

Check the Seven Basic Quality Tools.

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