spc-control-charts

# Statistical Process Control (SPC) ## When to use Use this skill when: - Selecting the correct control chart type for a process characteristic - Interpreting control chart signals — is this special cause or common cause? - Calculating Cp, Cpk, Pp, Ppk and determining if the process is capable - Responding to an out-of-control signal on a production line - Setting up SPC for a new special characteristic (PPAP/APQP requirement) - Auditing an SPC system for correctness and adequacy - Explaining SPC results to a customer or during an audit ## Prerequisites - The characteristic to monitor (variable or attribute data) - Production data (minimum 25 subgroups for control limits, 100+ pieces for capability) - MSA study completed and %GRR < 30% for variable charts - Process specification (nominal + tolerance) for capability calculations - Subgroup size decided based on rational subgrouping principle ## Workflow ### Step 1 — Select the correct control chart #### Variable data (measured values — length, weight, pressure, temperature) | Chart | When to use | |-------|------------| | **X̄-R (X-bar/Range)** | Subgroup size 2–9; most common in manufacturing | | **X̄-S (X-bar/Sigma)** | Subgroup size ≥ 10; better sensitivity to spread | | **I-MR (Individuals/Moving Range)** | Subgroup size = 1; one measurement per inspection (slow processes, destructive tests) | #### Attribute data (counts, pass/fail, defect rates) | Chart | When to use | |-------|------------| | **p-chart** | Prop...