Interviews of randomly selected service personnel regarding the quality of service that they provide would provide the least valid data for an audit of a retail store's customer service. This is because employees may not provide unbiased or entirely accurate information about their own performance. Additionally, self-reported data from employees lacks objectivity and may be influenced by personal biases or the desire to present themselves in a favorable light. Other options like customer surveys and graphs comparing staffing levels or training provide more objective and measurable data relevant to assessing customer service.