In 1993 Girobank decided to embark upon a mystery shopper exercise to provide information as input to its Total Quality management system. Girobank had already undertaken regular monitoring of its customer service through continuous customer surveys for several years, however, it was felt that these were not providing the complete picture that was required. In particular there was the need for an exercise that was essentially objective in nature and did not rely on the subjective perception of customers of a service that was distanced from them at the time of completing a survey. In addition mystery shopping would allow specific service aspects to be measured at key points in time. It would not be reliant upon individual customers' usage of service and the vagaries of human behaviour. Girobank's method of operation as a centralised bank with an agency relationship through the Post Office precluded any participation in syndicated mystery shopping exercises designed to evaluate retail branch banking per se. Key also to the evaluation of the service was the need to hold a Girobank account i.e. to be an existing Girobank customer. Girobank services are linked through a core customer relationship via the current account. Consequently we took the decision to utilise existing customers and invite them to participate in an ongoing exercise of service evaluation. This had a number of advantages: customers already knew Girobank and its method of operation - there was no need for them to undergo a learning curve on the service and the way it operates; accounts would not need to be opened or pre-funded - a consequent saving in costs, both in funds and administration time; existing customers would be invisible to customer service staff as 'mystery shoppers'; there would be no clue in their account details that they were a member of an evaluation panel, to all intents and purposes their behaviour would appear as natural; their calls would be indistinguishable from the thousands of calls received by Girobank daily. In addition, we could, within reason, pre-select customers on the basis of their existing account behaviour and product holding to better focus on those service aspects which we wished to evaluate.