Variable control charts

Learning objectives

• Explain the purpose of variable control charts in a garment-factory improvement project.
• Apply the described method to a representative shop-floor situation.
• Recognize the common mistakes and how to avoid them.

Concept

Variable charts monitor continuous measurements and separate within-subgroup from between-time variation.

I-MR suits individual observations; Xbar-R suits small rational subgroups (n≤10); Xbar-S suits larger.

Stabilize the process before assessing capability — capability on unstable data misleads.

Variables, units, and assumptions

Y in engineering units; subgroup size n (integer); time period.

Rational subgroups, gauge repeatable, independent draws.

Free-spreadsheet workflow

Free spreadsheet: subgroup means/ranges; UCL/LCL = mean ± A2·Rbar for Xbar; MR chart D3/D4 constants for range.

Interpretation and limitations

Points outside 3-sigma or Western Electric patterns indicate special cause.

Requires ~20-25 baseline subgroups; heavy skew inflates limits.

Garment-factory example

Monitor fused-panel bond strength in subgroups from each production interval, or daily average shade delta as individuals.

Method

1. Arrange in time order.
2. Calculate center and limits in a free spreadsheet.
3. Plot.
4. Check out-of-limit and nonrandom patterns.
5. Annotate causes.
6. Recalculate limits only after justified change.

Common mistakes

• Mixing unlike styles into one chart without stratification.
• Tampering after every common-cause fluctuation.