Causes of Non-normality of Monitored Quality Characteristics in Process Capability Analysis

Purpose: In process capability analysis, violation of the normality assumption is considered a non-standard situation. This paper defines and categorises causes of non-normality in manufacturing processes, which are essential for quality planning, control, and selecting appropriate analytical procedures. Based on a literature review and systematic investigation, four main categories have been identified. Each category is analysed in detail with practical implications for capability analysis. The paper provides a comprehensive framework for identifying non-normality causes and guiding further analysis steps. Methodology/Approach: Systematic literature review and expert synthesis of causes, supported by practical examples and categorisation into four key areas Findings: four categories of non-normality causes identified; each requires a specific analytical approach before capability analysis can proceed Research Limitation/Implication: Focuses on categorisation; future work should develop automated detection methods and validate solutions across diverse industries Originality/Value of paper: Provides the first comprehensive framework for identifying non-normality causes in manufacturing, bridging theory and industrial practice