The big picture of controlling quality is illustrated in the picture below:
Controlling quality involves monitoring specific results to determine whether they comply with the planned quality standards, which include project processes and product goals, and controlling the results by taking actions to eliminate unsatisfactory performance. In other words, the Perform Quality Control process is used to monitor and control quality by accomplishing the following goals:
• Monitor specific project results, such as cost performance and schedule performance, to determine whether they comply with the planned quality standards, which include project processes and product goals.The results under scrutiny include both deliverables and performance measurements by the project management team. Quality control is performed throughout the project. While dealing with quality control, you must be able to distinguish between the two terms in each of the following pairs:
• Identify ways to eliminate the causes of unsatisfactory performance.
• Prevention and inspection
• Prevention is a direction to perform an activity that will keep an error from entering the product and the process.
• Inspection is a technique to examine whether an activity, component, product, result, or service complies with planned requirements. The goal of inspection is to ensure that errors do not reach the customer.
• Attribute sampling and variable sampling
• Attribute sampling is a technique to determine whether a result conforms to the specified standard.
• Variable sampling is a technique to rate a result on a continuous scale that measures the degree of conformity.
• Common cause and special cause
• Common cause is a source of variation that is inherent to the system and is predictable. Such variations are also called normal variations, and the common causes for them are also called random causes.
• Special cause is a source of variation that is not inherent to the system and is removable. It can be assigned to a defect in the system.
• Control limits and tolerances
• Control limits is the area occupied by three standard deviations on either side of the central line or the mean of a normal distribution of data plotted on a control chart that reflects the expected variation of the data. If the results fall within the control limits, they are within the quality control.
• Tolerance is the range within which a result is acceptable if it falls within the limits of the range.
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