Work out control plan frequency instantly with clear inputs, formula shown and shareable results.
Control plan frequency should follow capability and consequence. A highly capable process (Cpk above 1.67) needs infrequent verification, while a marginal one needs frequent checks. The interval is then tightened where an escaped defect costs far more than an inspection, since the units produced between checks are all at risk if the process shifts.
Base interval from capability
Cpk >= 1.67: 4 h; 1.33-1.67: 2 h; 1.0-1.33: 1 h; below 1.0: 0.5 h
Cost adjustment
Interval = base x (20 / (defect cost / inspection cost))^0.25, bounded to 0.25-8 hours
Because 100% inspection is typically only 80-95% effective, costs a great deal, and does nothing to improve the process. Capable processes with periodic verification outperform incapable ones with full inspection.
They are mandatory regardless of interval. Setup is where most special-cause variation enters, and a last-off check bounds the quarantine if a problem is found.