Statistical process control using microbiological data

Statistical process control (SPC) is a system devised originally to improve industrial performance and control by graphic monitoring of results obtained on samples taken during processing. It led to a fundamental change in the philosophy of manufacturing control from one based on terminal inspection for faults (quality control) to continuous monitoring of process criteria to indicate if a process is going ‘out of control’ before it happens (quality assurance). Microbiological data on finished products and data gathered during verification of critical control points in a hazard analysis and critical control point (HACCP) scheme can be used to monitor both continuous and discontinuous processes. In addition, SPC systems can be used to monitor the standard of work in a microbiology laboratory and as part of retrospective analysis of problems such as assessment of probable causes of spoilage of products. Systems are described for variables, including traditional mean and range charts, cumulative sum (CUSUM) charts, CUSUM signal charts and moving windows charts, and for attributes; in addition, the use of ‘cause and effect’ diagrams is described. Numerous examples are used to illustrate how SPC charts can be prepared and used.