What is a Control Chart?

  • A trend chart with control limits
  • Graphical representation of process performance, where data is collected at regular time sequence of production
  • Valuable tool for differentiating between common cause and special cause variation
  • Evaluating whether a process is or is not in a state of statistical control
  • It lets the data ‘talk’ by itself & basis for data-driven decisions

Control Limits
A typical control chart consists of three lines :

control chart

control chart

CL: The average (measure of location) process performance when the process is in-control

UCL & LCL: The range of ‘usual’ process performance when the process is stable.  Lines drawn 3 standard deviations (3 sigma) on each side of the center line.

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