These deficiencies in documentation create one difficulty, but there are others. Ideally recruitment should be studied on the basis of three essential distinctions. First of all, there is the fundamental point of evolution over three centuries; it is obvious that Augustus did not find himself organizing his army in the same circumstances as one of the short-lived emperors of the third century who experienced 'the crisis of the empire'. Secondly, it would be preferable to be able to observe each of the main types of unit for this period - the garrison at Rome, the frontier armies and the fleet - as it is obvious that things did not happen in exactly the same way in each. A final point, it would be desirable to differentiate between centurions, officers of all ranks and ordinary soldiers in each type of unit. However, the gaps in our documentation as well as in the secondary literature do not allow a complete picture to be drawn.