Pre-control Chart

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Purpose

Pre-control charts are used as a conceptual aid in determining the state of a process and the action to take in the event of a change in the process.

This chart can be used to determine whether a process has changed and if corrective action is required, by comparing two successive units of process output against color coded Zones based upon the existing process’ Mean and Standard Deviation.


Anatomy

Pre-control Chart

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Reference: Juran’s Quality Control Handbook

Terminology

A. Pre-Control Chart for a two-sided process with output on either side of a Mean, or target, value

B. Pre-Control Chart for a one-sided process with a single specification or target, value

C. Green Zone – A three sigma wide band, centered on the process Mean, and extending 1.5 sigma above and below. Process output in the Green Zone is considered acceptable and the process may continue to run

D. Yellow Zone – A pair of bands, extending from +1.5 sigma to +3.0sigma and -1.5 sigma to -3.0 sigma. Process output in the Yellow Zone may permit the process to continue, or may require process adjustment or stoppage

E. Red Zone – A pair of open-ended bands extending above the +3.0 sigma point and below the 3.0 sigma point. Process output in the Red Zone is cause for immediate process stoppage and investigation

F. Green Zone for a One-Sided Process – A band extending from the Mean to -1.5 sigma for process with a lower specification (or+1.5 sigma for a process with an upper spec)

G. Yellow Zone for a One-Sided Process – A band extending from –1.5 sigma to –3 sigma (or +1.5 sigma to +3 sigma)
H. Red Zone for a One-Sided Process – An open-ended band extending beyond the 3 sigma limit


Major Considerations

The Pre-Control Chart permits quick determination of the state of a process, and the potential need for corrective action. Pre-Control can be used for 1 – or 2 – sided processes The use of Pre-Control requires that the process Mean is centered on target

Application Cookbook

1. Following the Improve Phase of a process change, collect sufficient data from the process to be able to determine the Mean and the Standard Deviation

2. Determine the Red, Yellow and Green Zone limits by calculating the points +/- 1.5 Sigma and +/- 3.0 Sigma from the Mean

3. Qualify the process by taking 5 consecutive samples in the Green Zone

4. Take two successive samples (A and B) from the process, and note their position relative to the color-coded bands on the Pre-Control Chart

  • Case Number 1 - Both A and B inside Green Zone - Take no action, continue
  • Case Number 2 - A is Green, but B is Yellow (Low) - Take no action, continue
  • Case Number 3 - A is Green, but B is Yellow (High) - Take no action, continue
  • Case Number 4 - A is Yellow (Low), B is Green - Take no action, continue
  • Case Number 5 - A is Yellow (High), B is Green - Take no action, continue
  • Case Number 6 - Both A and B are Yellow (Low) - Adjust process
  • Case Number 7 - Both A and B are Yellow (High) - Adjust process
  • Case Number 8 - A is Yellow (Low), B is Yellow (High) - Stop process
  • Case Number 9 - A is Yellow (High), B is Yellow (Low) - Stop process
  • Continue selecting pairs of samples from the process, per a pre-determined sampling schedule, and compare them per the Control Chart
  • Re-qualify the process after any adjustment or stoppage

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