Due to the intangible and inseperable of the services, and many services being the result of interaction between the service provider and the customer (coproduction), the performance of frontline employees is critical to customers perception about service quality. At the same time, the service delivery process has to be well designed, otherwise the flow of delivery may be called into question. However, we should know how to balance the tension between productivity improvement goals and standards of quality, inasmuch as from certain levels of productivity, quality may decrease.
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There is a recent paper that shows how to assess the association of employee engagement and customer engagement. They show the full scales for doing it as well. The citation is below. I hope you find it useful.
Kumar, V., and Anita Pansari. "Competitive advantage through engagement." Journal of Marketing Research (2016): jmr-15.