This is known as historiography. It is an important part of a high-level piece of History A-Level coursework. In brief, the aim is to explain the arguments of historians through the theory of history they employ and understand its limitations. For instance, the historian Christopher Hill is well known for using a Marxist approach to history, whereby trends in production and the economy define wider social changes. In comparing your historians, it is important to try and identify the limitations of their methodology: What or who was left out? In this way, you can form a conclusion which aims to find to common ground between the historians, and in doing add validity to your answers. In terms of structure, out of the 4000 words, dedicate around 800 words towards historiography, or around one detailed paragraph per historian.