Control charts II: SOP 17 and check standards as early-warning radar

The second procedure in the library's control-chart series, SOP 17, generalizes the idea beyond mass: any measurement process — volume transfer, length, an instrument bench — can carry a check standard, and the chart of its values is the process's flight recorder.

SOP 17's contribution is operational detail: how to establish limits from pooled data, what to do at a warning versus an action limit, and how the chart feeds the process's uncertainty budget with a live, honest Type A estimate rather than a repeatability number measured once in ideal conditions years ago.

SOP 30 then assembles the charts into a measurement assurance PROGRAM — the management layer that decides which processes carry check standards, who reviews the charts, and what 'out of control' triggers. The pair rewards reading together; both are page-searchable in the library, and the En-number article from earlier this month shows the same thinking applied between laboratories.