DNOM (Deviation from Nominal) Chart

The DNOM (deviation from nominal) chart is a SPC Chart PRO short-run chart — it lets many different parts share a single control chart when no one part runs long enough to chart on its own.

Short production runs are a common problem for control charting: a job shop or a flexible line might make a few pieces of one part, switch to another, then another, and collect too few observations of each part to establish a reliable separate baseline. DNOM solves this by plotting each measurement as its deviation from that part's nominal (its target dimension) instead of the raw value. Subtracting each part's nominal puts measurements on a common deviation scale. Zero represents the target; the calculated center line is the mean of the pooled deviations and can differ from zero.

When to Use It

Use DNOM when you run short batches of different parts and the parts share roughly the same variation — the spread of each part around its own nominal is about the same, even though the nominals differ. That is the usual case for a family of similar parts made on the same equipment.

If the parts have different amounts of variation, deviations from nominal are no longer on a common scale, and the Z-MR (standardized) short-run chart — which scales measurements using the selected sigma estimation method — is the better choice. DNOM is the simpler of the two and the right default when the equal-variation assumption holds.

How It Works

Your query returns one row per measurement, with a column that identifies the part and, optionally, a column giving each part's nominal. Consecutive rows of the same part form a run.

For each measurement, DNOM plots the deviation from nominal:

deviation = measurement − nominal

The panel charts these deviations using an individuals (I) chart and a moving-range chart, pooling the deviations across parts:

Chart Type (in the dropdown)	Plots
DNOM chart (deviation from nominal)	Each measurement's deviation from its part's nominal
mR chart (DNOM short run)	The moving range of consecutive deviations

Moving ranges are computed within a run only; they never span a part changeover.

Setting It Up

In the SPC section, with a DNOM chart type selected:

Option	Description
Part field	The field (text or number) that identifies each row's part. Consecutive rows of one part form a run. The field is hidden from the plot.
Nominal field	The numeric field giving each row's nominal (target). When no field is selected, each part's own mean is used as its nominal. Missing or non-finite nominal values also fall back to the part mean. The field is hidden from the plot.

Rows with a missing measurement or part identifier are treated as gaps. Estimating limits requires at least one pair of consecutive valid deviations within a run.

Add a DNOM panel for the deviations and an mR panel for the within-part spread, both pointing at the same query with the same Part field and Nominal field.

tip

If you don't have an explicit nominal for each part — for example, when you care only about consistency, not hitting a specific target — leave Nominal field empty and DNOM will center each part on its own mean.

Methods and Formulas

Let devᵢ = xᵢ − nominalᵢ be the deviation of measurement i, dev̄ the mean of the deviations, and MR̄ the average moving range of consecutive deviations within runs.

Chart	Center line	Control limits
DNOM (deviations)	dev̄	dev̄ ± 2.66·MR̄
mR (of deviations)	MR̄	D4·MR̄ and 0 (n = 2 constants)

The 2.66 and D₄ = 3.267 constants are the standard I-MR constants for a moving range of two — DNOM is an I-MR chart applied to the pooled deviations, with the one rule that a moving range never spans a part change.

info

The part and nominal columns are read as inputs and hidden from the plot. Capability indices are not shown for short-run charts, since the chart pools several parts. The part field, nominal field, and chart type are saved with the dashboard, and the chart works with both a time and a numeric X-axis.

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DNOM is part of SPC Chart PRO. See requirements, licensing, and support.