the development of mental models that reduce processing demands and help people to better prioritize their goals. In addition, it has been found that individuals vary in their ability to acquire SA; thus, simply providing the same system and training will not ensure similar SA across different individuals. Research has shown that there are a number of factors that make some people better at SA than others including differences in spatial abilities and multi-tasking skills. Criticism of model Early criticisms of the SA construct and the model have largely been unfounded and addressed. (1) "...models of SA refer to cognitive processes in general terms, but do not specify exactly what processes are involved and to what extent." However a detailed review by Parasuraman, Sheridan and Wickens found it has a strong body of empirical research, is diagnostic with regard to human states, is useful from a prescriptive standpoint, and has strong theoretical and empirical distinction from performance and other mental constructs. The Endsley model is very detailed in describing the exact cognitive processes involved in SA. (2) Others have questioned whether SA is circular, indistinguishable from performance. "The test of situation awareness as a construct will be in its ability to be operationalized in terms of objective, clearly specified independent (stimulus manipulation) and dependent (response difference) variables ... Otherwise, SA will be yet another buzzword to cloak scientists' ignorance.: This criticism however neglected the extensive work done to develop and validate objective measures of SA as a state of knowledge that are distinct from performance outcomes. A recent review and meta-analysis of objective SA measures found 171 studies that have successfully used these measures to quantify SA to evaluate system designs, training programs and investigate the construct of SA. (3) As a framework model, it draws on many constructs from cognitive science that need further definition themselves. Endsley addressed this criticism by stating that the model is very specific in its definitions and mechanisms, and that much subsequent research has supported the details of the model, including research on the role of working memory and long term-memory, cognitive mechanisms behind projection, individual characteristics affecting SA, the role of automation on SA, and the relationship between SA and workload . Related concepts Several cognitive processes related to situation awareness are briefly described in this section. The matrix shown below attempts to illustrate the relationship among some of these concepts. Note that