Control charts I: SOP 9 and the discipline of watching your process

A calibration certificate is a snapshot; a control chart is a film. NIST's SOP 9 — first of three control-chart procedures in the library — sets up the film for mass calibration: a check standard measured with the routine process, plotted against limits computed from the accumulated standard deviation.

The chart answers the question a certificate cannot: has anything changed since? A shift after a balance repair, drift from a deteriorating weight, a new operator's different technique — each shows up as points wandering past warning limits long before an external calibration would catch it. GLP 1 frames the same idea at the program level: quality assurance of measurement results means continuous evidence, not annual paperwork.

The statistics are deliberately plain — means, standard deviations, 2σ and 3σ limits — because the power is in the habit, not the math. Start with one check standard on the workhorse process; the later procedures in this series extend the technique.