Cheatsheet: Quality Control Basics

1. Quality Control Fundamentals

1.1 Key Definitions

1.2 QC vs QA Comparison

2. Statistical Process Control (SPC)

2.1 Control Charts

2.2 Types of Control Charts

2.3 Control Chart Formulas

2.3.1 X̄-Chart and R-Chart

2.3.2 p-Chart

2.3.3 c-Chart

2.4 Control Chart Constants

2.5 Out-of-Control Signals

• One or more points beyond control limits (±3σ)
• Seven consecutive points on one side of center line
• Seven consecutive points trending up or down
• Two out of three consecutive points beyond ±2σ warning limits
• Four out of five consecutive points beyond ±1σ
• Non-random patterns (cycles, systematic variation)

3. Process Capability Analysis

3.1 Process Capability Indices

3.2 Capability Index Interpretation

3.3 Key Relationships

• USL = Upper Specification Limit
• LSL = Lower Specification Limit
• μ = Process mean
• σ = Process standard deviation
• T = Target value
• If Cp = Cpk, process is perfectly centered
• Cpk ≤ Cp always
• For one-sided specifications, use Cpu = (USL - μ)/3σ or Cpl = (μ - LSL)/3σ

4. Sampling and Inspection

4.1 Sampling Methods

4.2 Acceptance Sampling Plans

4.3 Operating Characteristic (OC) Curve

• Plots probability of lot acceptance (Pa) vs. incoming quality (percent defective)
• Shows discriminatory power of sampling plan
• Steeper curve = better discrimination between good and bad lots
• Point at AQL should have Pa ≈ 0.95 (1 - α)
• Point at LTPD should have Pa ≈ 0.10 (β)

4.4 Sampling Plan Types

5. Six Sigma Methodology

5.1 Six Sigma Fundamentals

5.2 Sigma Levels and Defect Rates

5.3 DMAIC Process

5.4 DPMO Calculation

6. Quality Tools and Techniques

6.1 Seven Basic Quality Tools

6.2 Additional Quality Tools

6.3 FMEA Risk Priority Number

• Higher RPN = higher priority for corrective action
• Maximum RPN = 1000

7. Measurement System Analysis (MSA)

7.1 Key Concepts

7.2 Gage R&R Acceptance Criteria

7.3 Gage R&R Calculation

8. Continuous Improvement

8.1 Key Methodologies

8.2 Eight Wastes (DOWNTIME)

• Defects: Rework, scrap, incorrect information
• Overproduction: Making more than needed
• Waiting: Idle time, delays
• Non-utilized Talent: Underutilizing employee skills
• Transportation: Unnecessary movement of materials
• Inventory: Excess stock, work-in-progress
• Motion: Unnecessary movement of people
• Extra Processing: More work than required

8.3 PDCA Cycle Steps

9. Cost of Quality

9.1 Quality Cost Categories

9.2 Cost Relationships

• Cost of Good Quality = Prevention Costs + Appraisal Costs
• Cost of Poor Quality = Internal Failure Costs + External Failure Costs
• Total Quality Costs = Prevention + Appraisal + Internal Failure + External Failure
• External failure costs are highest and most damaging
• Investment in prevention reduces total quality costs

10. Key Statistical Formulas

10.1 Descriptive Statistics

10.2 Normal Distribution

• 68% of data within μ ± 1σ
• 95% of data within μ ± 2σ
• 99.7% of data within μ ± 3σ
• Control limits set at ±3σ from mean