📊 Statistical Process Control (SPC) Framework

SPC is a powerful method used to monitor, control, and improve processes using statistical analysis.
Key principle: If you measure it, you can improve it.

🔄 Process Boundaries: SIPOC Model

Suppliers ➡ Inputs ➡ Process ➡ Outputs ➡ Customers

Inputs

  • 👥 People Management
  • 📋 Methods (SOPs)
  • 📐 Specifications
  • 📂 QMS Documents
  • 🌿 Environment

Process

Central transformation stage where SPC is applied.

Outputs

  • 📦 Products
  • 🛠️ Services
  • 📝 Reports
  • 🔁 Change Management
  • 📞 Support Management

Feedback Loops

  • 🔁 Voice of the Customer: Drives improvement based on customer needs.
  • 🔁 Voice of the Process (SPC): Data-driven insight to monitor stability and capability.

📥 Data Collection Example

Weekly Data Collection Sheet

Time Day 1 Day 2 Day 3 Day 4 Day 5 Total Avg
8:00 AM0.250.250.270.250.311.330.266
10:00 AM0.380.300.340.320.281.620.324
12:00 PM0.280.280.270.300.301.430.284
2:00 PM0.300.280.270.320.281.450.290
4:00 PM0.260.270.280.320.321.420.284
Total7.311.462

📈 Control Charts & Limits

Control Limits (±3σ from the mean)

  • UCL: Upper Control Limit
  • LCL: Lower Control Limit
  • Center Line: Mean of process data

🔍 In a stable process, ~99.73% of data points fall within these limits.

🧪 Process Control Methods

  • 🔒 Mistake Proofing (Poka-Yoke)
  • 📄 100% Inspection (used in high-risk scenarios)
  • 📊 Statistical Process Control (preferred method)

📏 Process Capability

Formulas

  • Cp = (USL − LSL) / 6σ
  • Cpk = min[(USL − μ) / 3σ, (μ − LSL) / 3σ]

Interpretation

  • ✅ Cpk = 1 → Meets specs, but barely
  • ❌ Cpk < 1 → Not capable (produces defects)
  • 🚀 Cpk > 1 → Capable process (stable, low defects)

🔍 Cp measures spread only.
Cpk measures both spread and centering relative to spec limits.

✅ Final Notes

  • SPC captures the Voice of the Process and empowers data-driven decisions.
  • Use SPC with Lean and Six Sigma to build sustainable, defect-free operations.
  • Remember: Control before Capability.