A major retailer used shoppers to observe and record sales clerks' assistance, courtesy, and efficiency. This is an example o what type of observational research

This is covert observational research. It is most often used with the "secret shopper" concept as a way of testing quality control and customer service among associates. This makes sure that company directives and policies are being followed when it comes to transactions and resolving possible disputes, and that high-quality service is being given to each customer.