defectpercentages

Defect percentages are a quality metric that expresses the share of inspected units that contain at least one defect. They are typically expressed as a percentage of the total inspected units and calculated as (defective units / total inspected units) × 100. When a unit may incur multiple defects, organizations may report both the defect percentage (based on defective units) and the defects-per-unit metric, which counts defects rather than defective units.

Example: in a batch of 200 items, 14 units have at least one defect. The defect percentage is (14/200) × 100 = 7%. If those 14 units together account for 24 defects, the defects-per-unit would be 24/200 = 0.12 defects per unit.

Measurement can be based on 100% inspection or sampling. In sampling, the defect percentage is an estimate with confidence intervals and may require extrapolation to the full lot. Data collection should specify the sampling method, the criteria for defect identification, and the definition of what constitutes a defect.

Applications include manufacturing, electronics, food and pharmaceuticals, and software quality. A lower defect percentage generally indicates higher quality, but targets vary by industry and process capability. Defect percentage relates to other metrics such as yield (good units / total units), scrap rate (waste due to defects), and defects per million opportunities (DPMO), each requiring its own definitions of opportunities and units.

Limitations include that the metric does not capture defect severity, criticality, or whether multiple defects occur within the same unit. Best practices are to report defect percentage alongside defect counts, specify whether measurements are per unit or per defect, break down results by process step, and track trends over time.