Read SPC control charts

An SPC (statistical process control) chart shows whether a process is stable. It plots every value measured during inspections and draws the limits around them, so you can see drift or unusual results before they turn into rejected batches.

Before you start

The chart reads the Measured values entered in quality inspections for measurement criteria. For limits and capability, the criteria need a Min and a Max in their checklist.

Steps

1. Open Manufacturing › Quality › Quality Reports. The page SPC Control Chart opens.

2. Choose an item, or keep All Items.

3. Choose a Characteristic, or keep All Characteristics. A characteristic is a checklist criterion, such as "Coating thickness".

4. Choose the period: last 7, 30, 60 or 90 days.

Read the chart

• Measured Values: one point per measurement, in date order.

• Target: the midpoint between the criterion's minimum and maximum.

• The specification limits are the criterion's Min and Max: the range the customer accepts.

• The control limits are the average of the measurements plus and minus three standard deviations: the range the process normally produces.

• Out of Control: the number of points outside the control limits, highlighted on the chart.

• Samples: the number of measurements in the selection.

Process capability (Cpk)

Select a single characteristic whose criterion has both limits, and the Process Capability (Cpk) card shows how comfortably the process fits inside the specification.

• Capable when Cpk is 1.33 or more.

• Needs attention below 1.33.

Without a single characteristic, or without both limits, the card shows Select a single characteristic with spec limits to compute Cpk.

Common questions

The chart says there is no measurement data. Why? No inspection in the period recorded a measured value for the selection. Check that inspectors fill the Measured field for measurement criteria.

Can I enter measurements directly on this page? No. Measurements come from quality inspections only.