Statistical Process Control 101

Process Behavior and Control

The terms in control and out of control are typically used when referring to a stable or unstable process. A process is in control (stable) when the average and standard deviations are known and predictable. A process is out of control (unstable) when either the average or standard deviation is changing or unpredictable.

  • In control: Stable, predictable, consistent, unchanging
  • Out of control: Unstable, unpredictable, inconsistent, changing

In Control

An in-control process has many benefits:

  • Scrap and rework estimates can be made prior to production.
  • Machine settings can be adjusted to optimize throughput.
  • Engineers can incorporate statistical tolerance into their drawings, increasing component tolerances without compromising assembly performance.
  • Product designs can be statistically modeled to accurately predict fit and performance yields prior to prototype assembly.
  • Machine utilization can be optimized (e.g., high-precision machines and resources will not be wasted on manufacturing low-precision dimensions).
  • Process-improvement resources will be better spent.

Remember, being in control does not mean that the process is within specification. A process can be extremely stable while consistently producing bad product.

Out of Control

A process is usually judged to be out of control based on five commonly used control chart rules. These rules signal a change in either the process average or the variation.

  1. Points are beyond control limits.
  2. Eight or more consecutive points are either above or below the centerline.
  3. Four out of five consecutive points are in or beyond the 2-sigma zone (referred to as zone B in the graphic).
  4. Six points or more point in a row are steadily increasing or decreasing.
  5. Two out of three consecutive points are in the 3-sigma region (referred to as zone A in the graphic).

Even an out-of-control process can reveal useful information. By using SPC to measure out-of-control processes, you can do the following:

  • Detect both unwanted and desirable process changes.
  • Prove whether a process change resulted in an improvement.
  • Determine when to make a process change.
  • Verify measurement system improvements.

Control charts, sometimes called process behavior charts, are tools to determine whether a process is stable or unstable.

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