As data centers become increasingly complex and deliver services of high importance, it is very important that the quality of the delivered services can be objectively evaluated and can fulfill the expectations of the customers. In this paper, we present a novel, general, and formal methodology to determine and improve the Quality of Services (QoS) delivered by a data center. We use a formal mathematical model and methodology in order to calculate the overall indicator of the service quality and discuss methods of improving the QoS. Since the considerations were conceived and results have been proved in a formal model, the considerations and results also hold in a more general case. We discuss the pros and cons of the Continuous Change Strategy and analyze the Customer Dissatisfaction (CD) concepts. We show that CD is not the opposite of Customer Satisfaction (CS), but it can be used in a meaningful way to estimate CS. We introduce the queueing model and use the operation curve and the flow factor to improve the performance of data centers.