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Precontrol is a tool whose stakes are identical to the Shewhart control charts.


The Precontrol tool, also called stop lamp control and introduced in 1954, is used to track and identify a drift of the solutions implemented. And therefore indicate whether it is necessary to intervene and to make an adjustment.

This method will apply to the root cause, the Red X, to which we will define limit tolerances of specifications. It is based on the same principles as the control cards.

1-Definition of tolerances

Shainin proposes to identify tolerances using the technique of the parallelogram of tolerance also called Scatter plot. The technique is as follows:

  1. We carry out 30 measurements
  2. These 30 measurements are represented with the Red X value and y the Green y
  3. The linear regression line is drawn
  4. On both sides of the regression line, we draw 2 parallel lines in which the set of points must be contained
  5. We draw 2 horizontal lines that represent the maximum and minimum value of the Green Y
  6. We then draw 2 vertical straights which are at the intersection between the tolerances of the Green Y and the 2 straights of the parallelogram.
  7. The space between the 2 vertical straights and the x-axis represents the tolerance of the variable of the Red.

2-Definition of the control chart

From the tolerances defined previously, we will build the map of associated controls. It consists of 3 zones:

Bilateral tolerance

Unilateral tolerance

Green Zone

50% of the total tolerance on either side of the central axis.

86% of the total tolerance starting from point 0.

Yellow Zone

2 zones each representing 25% of the total tolerance and located between the Green Zone and the maximum and mini tolerances.

1 area that represents the remaining 14% of the tolerance zone.

Red Zone

Areas outside the specification limits.

3-Set up the rules

From there, we will be able to define the rules of reactions. According to Shainin, the rules are as follows:

during production: A 2-part sampling is carried out at regular intervals. If:

  • The two are in the green, we continue the production.
  • 1 is in the yellow and one is in the green: we continue the production.
  • The 2 are in the same yellow zone, so we intervene to readjust.
  • The 2 are each in a different yellow zone: we stop and settle.
  • 1 is in the red, so we intervene to adjust.

After setting: We test 5 parts, and the 5 must be in the Green Zone to validate the settings.


J. C. Whitaker (2005) – The Electronics Handbook

D. Shainin (1984) – Better than good old Xbarre and R charts asked by vendees

R. W. Traver (1985) – Pre-control: A good alternative to Xbarre R charts

A. A. Salvia (1988) – Stoplight control

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