Sample Size for Process Capability Studies: Minimum Requirements for Reliable Cpk
Sample size for process capability studies: the Cpk confidence interval formula, AIAG/ISO/Bosch standards, and worked numbers from n=30 to n=200.
Free SPC Software for Quality Engineers: Control Chart Tools Without Enterprise Pricing
Honest taxonomy of free SPC software for quality engineers — trials vs OSS vs browser tools — and which fits which use case without 30-day clocks.
Common Cause vs Special Cause Variation: Reacting Correctly to Control Chart Signals
Reading control chart signals correctly: tell common cause from special cause, map each Nelson rule to a likely cause, and act without tampering.
CUSUM vs Shewhart Charts: Detecting Small Process Shifts That X-bar Charts Miss
CUSUM vs Shewhart charts for small shift detection: 1-sigma shift takes 44 subgroups on Shewhart vs 10 on CUSUM. Decision framework + EWMA middle ground.
Gage R&R Before SPC: AIAG Acceptance Thresholds, Worked Example, and 5 Fixes When It Fails
Gage R&R workflow with AIAG %GRR thresholds (10/30%) and ndc criterion. Worked example computes %GRR = 30.5% and ndc = 4, plus what to do when MSA fails.
Histograms with Specification Limits: 7 Patterns That Tell You What Cpk Cannot
7 histogram patterns with spec limits and what they mean for process capability: centered, wide, off-center, bimodal, truncated, skewed, non-normal.
p-Chart Control Limits: Variable Sample Size, 3-Sigma Formulas, and Laney p-Prime
Step-by-step calculation of p-chart control limits with a worked PCB inspection example. Covers the variable sample size problem (exact, average-n, and standardized approaches), the np>=5 minimum, and the Laney p-prime chart for overdispersion.
Individual and Moving Range Charts: When Subgroups Are Not Practical
When and how to use the I-MR (Individual Moving Range) control chart. Covers the five scenarios requiring individual charts, a complete worked example with control limit formulas (d2=1.128, D4=3.267), normality assumptions, and sensitivity comparison vs. X-bar R.
Choosing the Right Control Chart: X-bar R, IMR, p, np, c, u Decision Guide
Decision tree for selecting the correct SPC control chart type. Covers variable vs. attribute data, subgroup size thresholds, p-chart and c-chart distribution requirements, CUSUM and EWMA alternatives, and common chart-type mismatches with real consequences.
Cpk vs. Ppk: Which Capability Index to Report and Why the Gap Between Them Matters
Comparison of Cpk and Ppk process capability indices with worked example, formulas, and a decision table for which index to report in PPAP submissions, customer audits, and ongoing SPC monitoring. Includes diagnostic use of the Cpk-Ppk gap.
Western Electric Rules and Nelson Rules: Choosing the Right Out-of-Control Tests for Your SPC Program
Decision framework for selecting between Western Electric Rules (4 tests, 1956) and Nelson Rules (8 tests, 1984) on SPC control charts. Covers false alarm rates, pattern recognition examples, and how to map each rule type to your reaction plan.
X-bar and R Chart Construction: Subgroup Size, Control Limits, and Reading Signals
Step-by-step worked example for building an X-bar and R control chart from CNC manufacturing data. Includes A2, D3, D4 constants for n=5, control limit formulas, pattern rule application, and Cpk calculation.