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 never collect the 20–30 points a normal chart needs for any single part. DNOM solves this by plotting each measurement as its deviation from that part's nominal (its target dimension) instead of the raw value. Because every part's deviations center on zero, parts with completely different nominal sizes plot together on one chart, and the run builds up the data a control chart needs.
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 divides each deviation by the part's own sigma — 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
and charts those deviations as an individuals (I) chart, paired with a moving-range chart — the same I-MR pair you would use for a single stream, but now over the pooled deviations of every part:
| 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 — the range across a part changeover is never taken, so switching parts does not create a false signal.
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 left empty, each part's own mean is used as its nominal. The field is hidden from the plot. |
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.
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.
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.
DNOM is part of SPC Chart PRO. For requirements, licensing, and support, see the SPC Chart PRO documentation.