Variables Control Charts

SigmaResolve recommends the appropriate chart type based on your subgroup size: I-MR for individual measurements (n=1), X-bar R for subgroups of 2-8, X-bar S for subgroups of 9 or larger. The recommendation appears as a suggestion you can override — experienced quality engineers may have reasons to use a different chart type.

Control limits use standard AIAG SPC Reference Manual constants. For X-bar R: UCL = X̄̄ + A₂R̄, LCL = X̄̄ - A₂R̄. R-chart: UCL = D₄R̄, LCL = D₃R̄. Constants are implemented for subgroup sizes 2-25. Control limits are calculated from your initial study period (configurable, minimum 25 subgroups recommended). Known special-cause points can be excluded from limit calculations with annotation.

Charts display A/B/C zones (1σ, 2σ, 3σ boundaries) with distinct shading to aid visual pattern recognition. Optional specification limit overlay shows USL and LSL as dashed red lines, visually distinct from control limits (solid blue lines) — both labeled with text to prevent the most common SPC mistake: confusing control limits with specification limits.

Lock control limits from your initial study and apply them to ongoing production. A visual indicator shows where limits were established vs. where they're being applied. Mark process changes (new tooling, material lot, machine adjustment) with vertical phase lines. Control limits can be recalculated per phase or carried forward — your choice.