Ppk Long-term Process Performance
Cpk uses within-subgroup std dev; Ppk uses overall std dev. A large gap indicates long-term drift or batch-to-batch variation.
An electronics plant tracked a new part over three months of mass production and found short-term Cpk of 1.45 but Ppk of only 1.12, revealing batch-to-batch drift. Ppk uses the overall (long-term) standard deviation to evaluate process performance. It is a key metric for supplier approval (PPAP) and SPC handoff, reflecting the true long-term quality level of shipped product.
Data: 100 samples (across 10 batches), overall mean = 10.02 mm Overall std dev σ_overall = 0.15, USL = 10.5, LSL = 9.5 Ppu = (USL - X̄) / (3σ) = (10.5 - 10.02) / (3 × 0.15) = 1.07 Ppl = (X̄ - LSL) / (3σ) = (10.02 - 9.5) / (3 × 0.15) = 1.16 Ppk = min(1.07, 1.16) = 1.07 → Below the 1.33 acceptance criterion — investigate batch-to-batch drift