Lean Six Sigma, CAPA Effectiveness and First-Time Quality in High-Mix, Low-Volume Manufacturing

Abstract

Purpose: Lean Six Sigma (LSS) is well established in high-volume manufacturing, but its effectiveness in high-mix, low-volume (HMLV) environments, where product variety and manual assembly complicate standardisation, is less understood. This study examines whether LSS-driven corrective and preventive action (CAPA) improves first-time quality (FTQ) in such settings and whether its effects are sustained over time.

Design/methodology/approach: A quasi-experimental design was applied to longitudinal production data from 14 assembly plants producing heavy vehicles, specialty machinery and industrial equipment in North America and Europe, covering 36 months, 840 production lots and 21,364 units. Seven plants that implemented a structured DMAIC-based CAPA programme in month 19 were compared with seven non-implementing plants using difference-in-differences estimation and interrupted time series analysis. Negative binomial regression modelled defects per unit (DPU), controlling for product complexity, production rate, workforce tenure, and plant size. Statistical process control charts assessed process stability. Results in this draft come from a simulated dataset with this structure.

Findings: Following implementation, FTQ increased by 8.1 percentage points, and DPU fell by 23% relative to comparison plants. Rework hours per unit declined by 18%, and recurrence of previously corrected defects dropped by 59%. Gains were largest for assembly and fastening defects (−38%) and absent for supplier-related defects. Improvements were sustained over 18 months, although the reduction in DPU stopped growing after 12 months. Higher product complexity weakened, but did not eliminate, the effect.

Practical implications: Managers in high-mix environments can expect substantial quality gains when CAPA is embedded in daily work rather than run as isolated projects, but should pair LSS with supplier quality programmes to address defects originating upstream.

Originality/value: The study provides multi-plant quasi-experimental evidence on LSS effectiveness in high-mix, low-volume manufacturing, a context largely neglected by research centred on high-volume automotive and electronics production, and distinguishes the immediate and sustained effects of CAPA.