In an experiment using the Patriot air defense training simulator, we found that the order in which the same information was presented to experienced Patriot officers significantly affected their probability estimates, identification judgments, and engagement decisions. In all cases the results represented a primacy effect, contradicting the recency effect predictions of the Hogarth and Einhorn (1992) model. In the present study we found that early in an aircraft's track history, the order effect is caused by the overweighting of prior information; later in the history, the late order effect is caused by participants reinterpreting the meaning of information based on what preceded it. These findings help to explain the differential effectiveness of a proposed display and the importance of naturalistic research for testing the generality of models of decision making.