Venice, Savoy, Denmark, Holland, Brandenburg, Saxony, and Weimar could not save them from our victorious arms."</ref> However, exact estimates are hard to calculate for various reasons. There are several sources for the study of the size of armies in different periods. Administrative sources By their own nature they are the more objective sources available. Since Napoleonic Wars European Commanders had at their disposal periodical strength reports of their units. Those strength reports are the main source for research in conflicts in 19th and 20th centuries, however they are not without problems, different armies count effective strength in different ways, and in some instances reports are inflated by commanding officers to look good to their superiors. Another source was muster calls, non-periodical strength reports of the personnel ready for duty. Muster calls are the main source for the strength of armies before the 19th century, but by their own nature they lack continuity and are ill-suited for long time period analysis. They are, however, the most reliable source for the period and do provide a general picture of army strengths and their variability. Thirdly, pay rolls provide another set of information. They are especially useful to study army costs, but they are not so reliable as muster calls as they only show payments, not real soldiers ready for duty, and before the 19th century "ghost soldiers", men falsely enlisted by officers in order to get the fees for themselves, were a very common occurrence. Finally, Orders of Battle, lists of units without specifying strength, are very important for the 16th, 17th and 18th centuries. Before that period armies lacked the organization to deploy permanent units, so that orders of battle usually consist in an enumeration of leaders with commands. The exception for Ancient Times would be the Roman army, that from an early period developed a considerable military organization. An Order of Battle is not a reliable source for army strength, since units in campaign, or even in peace time periods, are rarely if ever at full authorized strength. Narrative sources Modern historians make use of the large amount of administrative sources available now, however things were very different in the past. Pre-modern writers too many times give numbers without naming sources, and there are few cases in which we can be sure they are actually using any administrative source. That is especially true when they speak about enemy armies, in