Defect rate refers to the proportion of defects occurring, and it is commonly expressed either in percentage (%), or in parts per million (PPM). The mechanisms by which defects occur in the first place are due to factors such as errors in judgment by workers, equipment management/settings, setup, and environmental settings. The benchmarks for defect rates vary across industries in the manufacturing sector, but generally, the standard deviation ‘σ/sigma’ is used as a reference. Commonly, a control limit of ‘average ±3σ (99.7%)’ is employed as a stringent requirement for quality management. Manufacturers in various industries aim to reduce this defect rate through continuous improvement efforts on a daily basis.