Quality refers to the ability of a product or service to meet customer needs and expectations. There are several techniques used to ensure quality, including ISO 9000 quality standards, quality control processes, quality circles, and inspection processes. Control charts are used to monitor quality and identify when a process is out of control by plotting metrics like mean, range, defects per unit, or number of defects over time. Control charts can be for variables, to monitor measurable characteristics, or attributes, to monitor pass/fail outcomes.