Final answer:
Sales returns and allowances due to quality deficiency in a COQ report are classified as external failure costs.
Step-by-step explanation:
In a Cost of Quality (COQ) report, sales returns and allowances due to quality deficiency would be classified as external failure costs.
External failure costs represent the costs incurred as a result of product or service deficiencies that are identified after delivery to customers. This includes expenses like refunds, warranties, and customer support.