Quality Control

Quality control (QC) is the set of operational techniques used to measure, inspect, test, and monitor products and processes against specified requirements. The objective is not simply to find defective products after production, but to detect abnormal process behavior early and reduce the occurrence and escape of defects. Quality assurance (QA) is broader and focuses on establishing systematic processes that provide confidence that quality requirements will be met, while QC provides operational evidence through inspection, measurement, and testing. Statistical process control (SPC) uses statistical information from process measurements to distinguish common-cause variation from special-cause variation. Control charts such as X-bar, R, S, and p charts can be used depending on the type of data and sampling scheme. A process can be statistically stable but still incapable of meeting specification limits, so process capability indices such as Cp and Cpk provide additional information when their assumptions are appropriate. For a normally distributed process, Cp = (USL - LSL)/(6σ), while Cpk = min[(USL - μ)/(3σ), (μ - LSL)/(3σ)]. Acceptance sampling evaluates a lot using a sample rather than inspecting every unit. Its operating characteristic (OC) curve describes the probability of accepting a lot as a function of its quality level. AQL and LTPD are quality levels used in specifying sampling plans, while producer's and consumer's risks describe the probabilities of incorrect acceptance or rejection at designated quality levels. The PDCA cycle provides a structured framework for iterative improvement: Plan, Do, Check, Act. Common quality tools include check sheets, Pareto charts, cause-and-effect diagrams, histograms, scatter diagrams, and control charts. Quality costs are commonly grouped into prevention, appraisal, internal failure, and external failure costs. Effective QC therefore combines measurement, statistical analysis, process monitoring, root-cause investigation, and appropriate acceptance criteria rather than relying only on final inspection.