Quality
Cp / Cpk
Process Capability Index · cp · cpk · process capability
TL;DR
Short-term process-capability indices — Cp measures spread vs spec, Cpk measures spread vs the nearer spec limit. ≥1.33 is the usual minimum for a controlled process.
Cp = (USL − LSL) / 6σ — pure spread vs tolerance, assuming the process is centred. Cpk = min((USL − μ)/3σ, (μ − LSL)/3σ) — penalises off-centre processes. Standard customer expectations: Cpk ≥ 1.33 'capable', ≥ 1.67 'highly capable', < 1.0 'incapable'. Use short-term (within-batch) sigma — for long-term, use Pp / Ppk.
V5's SPC module calculates Cp / Cpk live per CQA and CPP, flags any drift below 1.33, and ties the alert to a deviation owner so the response is logged.
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