Module 4 · Lesson 13 of 22
Variance and standard-deviation comparisons
This lesson and the entire Lean Six Sigma Black Belt academy are free of charge — no fees, ever.
Learning objectives
- Compare spreads between groups.
- Choose robust methods when normality is suspect.
- Interpret variance ratios in operational terms.
Why variance matters
Two methods can share a mean but differ in spread. In garment conformance, tighter spread often matters more than a slightly better mean because tolerances are two-sided.
Chi-square and F methods are sensitive to normality; Levene / Brown-Forsythe methods are more robust.
Garment-factory example
Two sewing methods have equal mean seam allowance but Method B has σ = 0.6 mm versus Method A σ = 1.1 mm — Method B is preferable because both spec limits are within reach.
Method
- Plot spread by group and by time.
- Check stability before comparison.
- Use a robust test in a free tool.
- Report ratio and CI; investigate mechanism.
Common mistakes
- Comparing σ on unstable, mixed data.
- Claiming lower spread without an uncertainty range.
Knowledge check
Pick one answer per question. Explanations appear after you submit.
1. Equal means with unequal spreads:
2. Levene / Brown-Forsythe is used because:
Author: NOSK.life · Last reviewed: 2026-07-21