Thompson points out that in some cases, as for instance in his account of British actions, Ammianus could not use his sound historical principles (20.1.1-3). He could not find enough good sources to cross-examine. The thinness of his account we can therefore attribute to Ammianus problem getting a good informative source, not to any sympathy or antipathy. Even when Ammianus had good sources, he did not always tell the story objectively, either because he wanted to highlight characteristics of a person or give his interpretation of an occurrence. Yet even Austin allows that Ammianus was not malevolently obsessive in his biases and had a good idea of how information was to be gathered and analysed.
Ammianus understood the gathering and handling of information well. He held a military position as protector domesticus (15.5.22). Protectores were an elite corps of officers, recruited from men of good family or with good prospects of promotion because of their abilities. Ammianus was probably a member of the curial class of Antioch, and his abilities would have been intellectual rather than physical. In his Res Gestae for example, Ammianus only once engages in a man to man fight, right before the fall of Amida (18.8.9ff). Ammianus, as a protector domesticus, worked in the headquarters of different high officers, such as Ursicinus. As a protector, he learned to analyse information for strategic and tactical purposes. According to Austin, this had an enormous impact on Ammianus Res Gestae.
Because of Ammianus excursus on artillery (23.4), some scholars have suggested that Ammianus was an artillery officer. Austin proves convincingly that Ammianus knowledge on artillery is a lot less substantial than his knowledge on gathering and evaluation of intelligence. Austin stresses that Ammianus knowledge of all kinds of war-machines will more likely have come from his interest and eyewitness observations than from a specialist training.
Ammianus knowledge of strategic and tactical purposes has been studied intensively. Ammianus knowledge of the divisions of strategy (intelligence gathering, planning and operations) and their interrelation seems to have been accurate and detailed. Austin points out that this is probably because Ammianus had much experience with exactly this kind of work in his position as protector domesticus.
On the other hand, Ammianus knowledge of the different methods that could be used to collect strategic intelligence seems limited. According to Austin, it is likely that there simply was a lack of sophistication in Roman techniques when it came to gathering information. Another discrepancy between Ammianus job and his Res Gestae seems to be that Ammianus did not describe the process of interpretation and evaluation of intelligence, although this was probably also an important part of his job. Ammianus might have left this part of strategic intelligence out because it was per se not a phase that would interest his readers. It was simply too technical and boring.