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High availability is driven by two types of factors: customer site factors such as the frequency of software and hardware upgrades, and system factors such as failure and repair rates, most often associated with mathematical models of reliability and availability. In this paper we describe several tools to assess the effects of these factors on the availability of high availability transaction processing (HATP) systems and make the expected level of performance more understandable to customers who purchase such systems, sales people who sell them, and managers who must make decisions based on the system availability. We employ a survey methodology to identify the key customer site factors that drive availability; we illustrate an accurate but greatly simplified technique for modeling system availability based on the internal system factors; and we apply statistical design of experiments and control chart methodologies to better understand the variability inherent in the system performance