The hierarchy of costs is not the only approach organizations use to group costs. Managers are also concerned about measuring the costs associated with quality. Quality-related costs can be organized into four categories. The first two categories-prevention and appraisal-are costs incurred to control and improve quality. The final two categories-internal failure and external failure-are costs incurred as a result of failing to control and improve quality. What is the difference between these cost categories, and how does this information help managers improve quality?