Short-Run SPC

Traditional SPC requires large subgroups and long production runs to establish control limits. Short-run SPC adapts control charting to low-volume production, job shops, and mixed-model lines.

The challenge

With 10 or fewer parts in a lot, you cannot calculate meaningful traditional Xbar-R control limits. Different part numbers have different nominal values and tolerances, making it impossible to plot them on a single traditional chart.

DNOM (Deviation from Nominal) chart

Plot (xᵢ − nominal) instead of xᵢ. If parts have the same expected standard deviation, all part numbers can be plotted on one chart. Example: Part A nominal = 25.000 mm, measured = 25.043 mm DNOM = 25.043 − 25.000 = +0.043 mm Part B nominal = 50.000 mm, measured = 49.972 mm DNOM = 49.972 − 50.000 = −0.028 mm Both plotted on the same deviation chart.

Standardized charts

When parts have different standard deviations, standardize the deviation by dividing by the target standard deviation:

Standardized value = (xᵢ − nominal) / σ_target Now all parts have target mean = 0 and std dev = 1, regardless of part size or tolerance.

CQE exam tips

DNOM requires that all part numbers have approximately the same process standard deviation.
Standardized charts work for any combination of parts but require an established σ for each part.
Short-run SPC is common in aerospace and defense job shops where lot sizes of 1–25 units are typical.

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