Precontrol Chart: Fast Go/No-Go Judgment for Short Runs
What is the Precontrol Chart?
Precontrol uses specification limits instead of statistical control limits: the distance from the specification center to the limit is divided so the green zone is the central half of the tolerance, yellow zones the next quarter on each side, and red beyond the specification. The P-line is the green boundary, halfway from the specification center to the limit. Consecutive sampling decides: green passes, yellow warns, red stops the line — directly judging product conformance without accumulating history.
When to Use It
Use it for short batches, frequent changeovers and low volumes where conventional control charts cannot be built, for pilot runs and new-product introduction, and for rapid patrol checks on small-batch, multi-product lines. It works as a quality gate within minutes; once volume stabilizes, the process can transition smoothly to conventional control charts.
How to Use It (Step by Step)
Set the specification limits and sample consecutively: two consecutive points in yellow on the same side, or any red point, means stop and adjust; five consecutive green points mean the process is stable and production continues. The tool draws the three zones and states the required shop-floor action, turning the judgment directly into an operating instruction.
Key Formulas / Example
The P-line sits at the midpoint between the specification center and the specification limit; green covers the central 50% of the tolerance, yellow the outer quarter on each side, and red lies beyond the specification. Precontrol does not estimate CpK — use the CpK tool for quantified capability.